Inorganic anisotropic hollow fibers

ABSTRACT

Essentially inorganic monolithic hollow fibers having a radially anisotropic internal void volume wall structure. Process for the production of such fibers. Apparatus and processes using such fibers.

This is a division of application Ser. No. 009,579, filed Feb. 5, 1979,now U.S. Pat. No. 4,268,278, which is a division of application Ser. No.906,502, filed May 16, 1978, now U.S. Pat. No. 4,175,153, issued Nov.20, 1979.

FIELD OF THE INVENTION

This invention relates to inorganic anisotropic hollow fibers, processesto produce such fibers and apparatus and processes that use such fibers.These fibers are useful in many fields, such as fluid separations, fuelcells and catalysis. They are particularly amenable to applicationsinvolving gas diffusion, e.g., hydrogen diffusion.

Separating fluids from fluid mixtures is an especially importantprocedure in the chemical processing industry. In order for theseparation of a desired fluid by the use of separation membranes to becommercially attractive, the membranes must be capable of withstandingthe conditions to which they may be subjected during the separationoperation and must provide an adequately selective separation of thefluid together with a sufficiently high flux, i.e., permeation ordiffusion rate per unit surface area, so that the use of the separationprocedure is on an economically attractive basis. Thus separationmembranes which exhibit adequately high selective separation, butundesirably low fluxes, may require such large separating membranesurface area that the large scale commercial use of these membranes isnot economically feasible.

It is known that hydrogen may be separated and purified from a gaseousmixture containing hydrogen and other gases by allowing the hydrogen topermeate selectively, at elevated temperatures, through thin non-porousnoble metal barriers. In this process, hydrogen under pressure isbrought into contact with one side of such non-porous barriers. Theother side of the barrier is maintained at a lower hydrogen partialpressure. The hydrogen diffuses through the barrier and is recovered inpurified form.

Among the factors on which the diffusion of hydrogen per unit areathrough such barriers depends are the thickness of the barrier, thepartial pressure differential between the high and low pressure sides ofthe barrier, the temperature of the barrier and the material from whichthe barrier is made.

Although the diffusivity of a barrier, i.e., the ability of the barriermaterial to permit a particular gas to diffuse therethrough, does notdepend upon the thickness of the barrier, the rate of diffusion isinversely proportional to such thickness. Since high diffusion rates areessential for the commercial feasibility of such barriers, it isnecessary that the barrier be as thin as possible, consistent withstructural stability under commercial operating conditions, and that itprovide a sufficiently large surface area for diffusion. Considerableefforts over a long period of time have been expended in attempts todevelop such thin barriers having large surface areas which will sustainsuch high diffusion rates while withstanding operating conditions. Theseefforts have extended over at least half a century. For instance,Snelling in U.S. Pat. No. 1,174,631 described a process for utilizing ametal, such as palladium or platinum, film maintained at an elevatedtemperature and supported by a base of porous earthenware or alundum.Snelling also described such a film supported on a porous cylindricaltube.

Other workers have utilized thin metal barriers supported on backingssuch as porous metal, ceramic, screen guards or other suitable materialto preclude distortion or collapse of the thin metal barrier.Difficulties have arisen with such thin barriers for hydrogen diffusion.For instance, attempts have been made to produce large surface areabarriers of about 1 mil (about 25 microns) in thickness by rolling,vapor deposition, and electroplating, however, these barriers haveproved to be troublesome if not unsatisfactory. Such barriers aredifficult to fabricate by rolling without pin holes with result inunsatisfactory performance as a separating barrier. Other proceduressuch as for instance by vapor deposition and electroplating areextremely slow and impractical.

Significant efforts have been expended in attempts to provide supportedplanar metal barriers which would provide commercially feasible hydrogendiffusion devices. See, for instance, U.S. Pat. Nos. 2,958,391,3,208,198, 3,238,700, 3,344,582, 3,344,586, 3,350,846, 3,413,777 and3,499,265. These efforts do not appear to have resulted in commerciallyadvantageous hydrogen diffusion devices.

It has also been proposed to utilize elongated tubes (which may becoiled) which do not require a separate support. These tubes can beprovided either singly or in multiple bundles in order to increase thesurface area for diffusion. Such bundles of tubes are illustrated, forinstance, in U.S. Pat. No. 2,961,062 of Hunter et al utilizingpalladium-containing capillary tubes which are described as being drawnto wall thicknesses of from about 25 microns to 126 microns, with a borediameter of from 794 microns to 3,175 microns. These tubes appear tohave "dense" or "compact" walls, i.e., an isotropic wall structure.Although the capillary tubes of Hunter et al may provide technicallyfeasible hydrogen diffusion cells, the practical limitations of drawingtubes of such diameters and wall thicknesses result in devices that areextremely expensive to produce. This is due both to high cost ofpalladium and the tube drawing procedures. Because of this expense it isextremely important that the tube drawing procedure produce tubes whichare substantially satisfactory for use with limited margin for errorwhich will result in loss of materials. That is, the wall thicknessesutilized must be satisfactory for both structural support and to avoidflaws that may allow gases other than hydrogen to pass through thebarrier. Although it is known that smaller tubes will enable the use ofthinner walls (because the inherent geometry of smaller tubes providesequal strength with thinner walls) it has been difficult to produce suchsmaller tubes which have wall thicknesses commensurate with the desiredoperating conditions. This is due to the practical limitations of smalltube production by tube drawing procedures and the prohibitive costsinvolved. Other workers have investigated the use of small tubes havingdimensions similar to those of Hunter et al. See, for instance, U.S.Pat. Nos. 2,911,057, 3,238,700, 3,172,742, 3,198,604, 3,208,198,3,226,915, 3,278,268, 3,392,510, 3,368,329, 3,522,019, 3,665,680 andBritish Pat. No. 1,039,381. All of the tubes utilized in the discloseddevices appear to have isotropic wall structures (barring flaws). Theseworkers have not suggested the use of smaller tubes or tubes havingwalls that are not isotropic. To date, metal tubes possessing strongeconomic potential have been elusive.

The present invention provides barriers that readily meet thisobjective. In addition to discovering thin metal barriers which arehighly suitable as the barrier components in, e.g., economicallyfeasible hydrogen diffusion apparatus, it has also been discovered thatthese components are useful in apparatus and processes in many otherapplications. Of particular interest is the broad field of fluidseparations by membranes.

The use of polymeric hollow fibers as separation membranes in variousfluid separation procedures is well recognized as having greatadvantages over planar membranes. This is due to the inherent geometryof the hollow fibers which provide a large membrane surface area forseparation within a unit volume of the apparatus containing them.Furthermore, such hollow fibers are known to be able to withstandgreater pressure differentials than unsupported planar membranes ofessentially the same total thickness and physical structure.

More recently, polymeric hollow fibers useful in fluid separations havebeen provided which have a so-called "Loeb-type" wall structure. Thisterm derives from the work of Loeb et al who found that, with planarmembranes, by using particular preparative techniques, they couldgreatly increase the water permeability through cellulose acetatemembranes. U.S. Pat. Nos. 3,133,132, 3,133,137 and 3,170,867 describethis method which results in what has subsequently been termed a"modified" membrane structure. This polymeric structure has beenextensively studied using differential dyeing techniques as well aselectron microscopy. Unlike previous commercial cellulose acetatemembranes, which appeared to be fully dense and without void structure,the membrane formed by the casting procedure of Loeb et al has been saidto have a void containing region and a separate dense region. The porousregion usually extended from the surface which was adjacent to thecasting surface during formation through approximately 90-99% of thetotal membrane thickness. The remaining "dense" region extends to theopposite surface. In other words, since the membranes are not ofessentially the same density throughout their thickness, they are deemed"anisotropic", i.e., they have distinct differences in void volume indifferent regions of the membrane thickness.

Other workers extended this anistropic structure to polymeric hollowfibers. See, for instance, U.S. Pat. Nos. 3,674,628, 3,724,672,3,884,754, and 4,055,696.

These anisotropic polymeric hollow fibers have been used as supports forseparation membranes or as the separation membrane itself.Unfortunately, although these polymeric hollow fibers have been used indesalination procedures and may provide excellent separation propertiesthey are often subject to limited usefulness and/or deterioration ofsuch properties due to their operating environments. For instance,numerous chemicals as well as undesirable chemical contaminants inliquid and gaseous streams may cause undesirable reactions with thepolymeric materials. Likewise, higher temperatures and pressures areoften incompatible with maintaining the desired properties of suchpolymeric fibers. Furthermore, these polymeric hollow fibers do notapproach the selectivity of the noble metal barriers.

Porous glass hollow fibers have been suggested as supports for permeablemembranes as well as the separation membrane itself. See for instance,U.S. Pat. Nos. 3,246,764 and 3,498,909. Such glass hollow fibers appearto have an isotropic internal void volume wall structure.

Although numerous procedures have been suggested for preparing inorganicfibers (see, for instance, U.S. Pat. Nos. 3,321,285, 3,311,689,3,385,915, 3,529,044, 3,565,749, 3,652,749, 3,671,228, 3,709,706,3,795,524, 3,846,527, 3,953,561, 4,023,989, 4,060,355, and 4,066,450) itappears that there has been no suggestion of the inorganic anisotropichollow fibers of the present invention.

In the description of the present invention, the following definitionsare used.

The term "hollow fiber" as used in this application means a fiber (ormonofilament) which has a length which is very large as compared to itsdiameter and has an axially disposed continuous channel which is devoidof the material that forms the fiber (more commonly referred to as the"bore"). Such fibers can be provided in virtually any length desired forthe use intended.

The term "internal void volume" is used to denote space included withinthe fiber wall devoid of the material that forms the fiber.

A region in the fiber wall is said to be a "compact layer" when it isrelatively dense (having substantially less and often virtually nointernal void volume) and is located in barrier-like relation to fluidflow through the wall. It may be either porous or essentiallynon-porous. The term "porous" refers to that characteristic of a compactlayer which, although otherwise being continuously relatively dense, hasvery small, often tortuous, passageways that permit the passage of fluidthrough the compact layer other than by diffusion.

The term "skin" is used to denote a compact layer that is at an internaland/or external surface of the fiber.

The term "peripheral external zone" is used to denote the externalregion of the fiber wall, the thickness of which is one-quarter toone-half the distance separating the external surface of the fiber fromthe internal surface, it being understood that this external region ofthe fiber may optionally be covered by a skin.

The term "peripheral internal zone" is used to denote the internalregion of the fiber wall which surrounds the bore, the thickness ofwhich is one-quarter to one-half the distance separating the internalsurface of the fiber from the external surface, it being understood thatthis region surrounding the bore may be separated from the bore by askin.

The phrase "essentially inorganic materials" denotes a sinterableinorganic material this is substantially free of organic polymericmaterial.

The term "monolithic" means that the material of the fiber has the samecomposition throughout its structure with the fiber maintaining itsphysical configuration due to the bonding between the sinteredparticles.

The phrase "radially anisotropic internal void volume" means that thevoid volume within the fiber wall varies in a direction perpendicular tothe axis of the fiber.

SUMMARY OF THE INVENTION

The present invention provides essentially inorganic, monolithic hollowfibers having a radially anisotropic internal void volume wallstructure. Preferred forms of such fibers are those that have a porousor essentially non-porous compact layer. Such fibers comprising metaland having an essentially non-porous compact layer are particularlypreferred.

The present invention also provides a process for producing such fiberscomprising (a) preparing a solution of an organic fiber-forming polymercontaining, in uniformly dispersed form, a sinterable inorganicmaterial; (b) extruding the inorganic material-containing polymersolution through a hollow fiber spinneret; (c) forming a precursorpolymeric hollow fiber laden with the inorganic material and having aradially anisotropic internal void volume wall structure; (d) treatingthe precursor polymeric hollow fiber to remove the organic polymer; and(e) sintering the resulting inorganic material; provided that steps (d)and (e) are conducted under conditions that maintain a radiallyanisotropic internal void volume wall structure in the hollow fiber. Apreferred form of the process is where the inorganic material isoxidized or reduced to a sinterable inorganic material during or priorto sintering. The essentially inorganic hollow fiber produced will havea wall structure which substantially correlates with the wall structureof the precursor polymeric hollow fiber but on a reduced scale due toshrinkage.

This invention also provides improved processes and apparatus employingsuch fibers. For instance, metal fibers having an essentially non-porouscompact layer are particularly useful in improving processes andapparatus involving gas diffusion. This is particularly advantageous forboth the production of hydrogen in substantially pure form and toeconomically shift equilibrium reactions which involve hydrogen. Stillother processes and apparatus will advantageously employ fibers of thisinvention, both with or without compact layers, as supports forinorganic membranes and/or polymeric membranes. The hollow fibers ofthis invention are also useful in improving processes and apparatus forfuel cells and in other catalyzed reactions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a photomicrograph of a cross section of a polymericprecursor hollow fiber containing as the metal component a mixture of50% nickel oxide and 50% iron oxide, both by weight, having a radiallyanisotropic internal void volume wall structure.

FIGS. 2 through 4 show photomicrographs of cross sections (or portionsof a cross section) of hollow fibers having a radially anisotropicinternal void volume wall structure that has a compact layer at thefiber's external surface (FIG. 2), internal surface (FIG. 3) and withinthe wall structure (FIG. 4).

FIG. 5 shows an end view of a small bundle of metal fibers of thisinvention sealed together inside a sleeve (magnification 50).

FIG. 6 shows a photomicrograph of the external surface of a hollow fiberof this invention showing a skin that is uniformly porous.

FIG. 7 schematically illustrates a hydrogen diffusion device whichcontains hollow fibers of this invention.

FIG. 8 schematically illustrates a hollow fiber of this invention andsilver tubing arranged for use as the electrode elements in a fuel cell.

FIG. 9 schematically illustrates, in cutaway fashion, a fuel cellcontaining the electrode elements illustrated in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hollow Fiber

The essentially inorganic, monolithic hollow fibers of this inventionhave a radially anisotropic internal void volume wall structure. Theyhave unique properties characterized by large surface areas (both withinthe wall structure and at the internal and external surfaces), readyaccess to these surface areas, and the ability to withstand hightemperatures and pressures and difficult chemical environments. Thefibers provided by the present invention are a major contribution toworkers in numerous fields; for instance, in fluid separations bymembrane (both as supports for separation membranes and as the membraneitself) and fuel cells, etc. These hollow fibers can be preparedrelatively economically with widely varying physical configurationswhile utilizing many types of inorganic materials. Furthermore, it hasbeen found that large amounts of these fibers can be produced at lowcost with only nominal losses due to flaws and imperfections.

The fibers of this invention comprise essentially inorganic materialswhich are sintered in hollow fiber form having the desired wallstructure. The sinterable inorganic materials comprise a very largegroup of materials. The preferred sinterable inorganic materials aremetals. Particularly preferred are the hydrogen diffusible metals suchas the noble metals, nickel, etc. and their alloys. Iron and its alloysare particularly useful. Nickel and its alloys, i.e., iron, are the mostpreferred metals. The sinterable inorganic materials can be ceramics,such as aluminum oxide, β-alumina, etc. The sinterable inorganicmaterials can also be cermets or metcers, such as iron metal/aluminumoxide, nickel metal/titanium carbide, etc.

These fibers have a radially anisotropic internal void volume wallstructure. In other words, where one region of the fiber wall may have arelatively high void volume, say, for instance, in the peripheralinternal zone, another region of the fiber can have a substantiallylower void volume say, for instance, in the peripheral external zone.These contrast with previously-known apparently isotropically porousinorganic hollow fibers (i.e., glass) which have substantially the samevoid volume throughout all regions of the fiber wall and the noble metaltubes which have isotropic dense or compact wall structures. The uniqueinternal void volume of the fiber wall structure of the fibers of thisinvention at any particular radii (perpendicular to the fiber axis) fromthe center of the hollow fiber, may be essentially uniform. In otherwords, when such fibers have concentric bores, generally the internalvoid volume at all points in the wall on any cylindrical ringconcentrically located around the fiber axis is substantially the same.

Fiber shapes other than circular are contemplated. For instance, havinga square, hexagonal, star or oblong shape or with fins, etc. Such shapescan be influenced by the spinneret design utilized and the fiberextruding and forming conditions.

In general, the overall internal void volume (meaning that volumeencompassed by the nominal internal and external surfaces of the fiber)can range from about 15 to about 95%. A preferred range of internal voidvolumes is from about 45 to about 90%. Fibers having an internal voidvolume in the peripheral external zone of from about 10 to about 35% andan internal void volume in the peripheral internal zone of from about 75to about 95% are particularly desirable.

As mentioned above, these fibers have large surface areas. For instance,due to their relatively small outer diameters the diffusion surface areaprovided per unit volume is extremely large.

These fibers also have particularly large and useful surface areaswithin the wall structure. Since it is quite possible that the inorganicmaterial may act in a dual capacity as both the supporting and/orfunctional structure of the fiber and as a catalytic material that willcatalyze reactions contiguous to the fiber surfaces these availablesurface areas within the fiber wall can provide very significantadvantages.

These fibers generally have an outer diameter of up to about 2,000microns. However, fibers of larger outer diameters, such as 3,000 or4,000 up to about 6,000 microns, are also contemplated. Such largerfibers may have to have thicker walls and would provide less activediffusion surface area per unit volume or may require a sacrifice in thepossible operating conditions. More preferred fibers have an outerdiameter of from about 50 to about 700, most preferably from 100 to 500,microns. The wall thickness is dependent on the bore size desired toavoid excessive pressure drop. The fibers often have wall thicknesses offrom about 20 to about 300 microns. More particularly preferred arefibers having wall thicknesses of from about 50 to about 200 microns.The fibers generally have a wall thickness to outer diameter ratio offrom about 0.5 to about 0.03, particularly preferred of from about 0.5to about 0.1.

It should be understood that the structures of the walls of the fibersof the present invention are not equivalent to the walls of the noblemetal tubes used in prior hydrogen diffusion processes due to the uniquevoid volume characteristics. Accordingly, direct comparisons betweenwall thicknesses of such noble metal tubes and the wall thicknesses ofthe hollow fibers of this invention are inappropriate. Rather, since thewalls of such tubes are substantially dense or compact with essentiallylittle or no internal void volume they could more appropriately becompared to the essentially non-porous compact layer of the fibers ofthis invention which actually represents the portion of the wallthickness actually participating in the diffusion.

The fibers of this invention can have a compact layer which may beporous or essentially non-porous. The thickness of the compact layer isless than 50%, preferably less than 30%, more preferably less than 15%of the wall thickness. When referring to the essentially non-porouscompact layer the thickness of the compact layer is convenientlyexpressed as the "effective thickness". This thickness being thethickness calculated from the actual amount of gas diffusing through theessentially non-porous compact layer and fiber wall and the intrinsicpermeability of the material of the fiber. For this determination, thefiber could be tested with another gas to assure the presence of anessentially non-porous compact layer. With porous compact layers, thethickness can be estimated by, for instance, scanning electronmicroscopy. In general, for fibers having outer diameters up to about1,000 microns, the compact layer thickness will be within the range offrom about 2 to about 80 microns, e.g., about 4 to 60 microns, and morefrequently about 10 to 50 microns.

Fibers having compact layers are particularly useful in gas separationswhere, for instance, with certain metals it is desired that onlyhydrogen diffuses through the essentially non-porous compact layer. Thecompact layer can be a skin at the external or internal fiber surfacesor can be within the fiber wall. A more preferred embodiment of theinstant invention is a hollow fiber having a skin (as defined herein) ona peripheral external, or on a peripheral internal, zone (as definedherein), or both; the zone or zones comprising a network of mutuallyintercommunicating internal void volumes that become progressivelylarger or smaller in a radial direction when traversing from one zone tothe other.

Particularly important fibers of this invention are those havingrelatively thin compact layers as a skin at the external fiber surface.Such fibers are very useful in fluid separation by membrane processes,for instance, in hydrogen diffusion processes. These fibers can act assupports (where the skin is porous) or as the membranes themselves(where the skin is essentially non-porous). They can exhibit adequatestrength under high temperatures and/or pressures. Exemplary of fibershaving a thin compact layer are metal, for instance, nickel alloy,fibers having a porous or essentially non-porous skin at their externalsurface which is from about 2 to about 40 microns thick, a wallthickness of from about 75 to about 125 microns and an outer diameter offrom about 250 to about 700 microns.

It is well known that, as the outer diameter of a tubular shapedecreases, the strength provided by a given wall thickness increases.Since fibers are now provided by the instant invention with relativelysmall outer diameters the wall thickness necessary for adequate strengthis reduced. This provides tremendous advantages in numerous applicationsbecause of the much higher active diffusion or permeability surface areaper unit volume available and the improved diffusion rates realized withthin walls and very thin skins. Furthermore, since such thin walls andvery thin skins are now a viable alternative it is possible to useinorganic materials, i.e., nickel and its alloys, not previouslyconsidered practical due to their lower intrinsic permeabilities. Thisprovides an improvement in cost, an improvement in strength and amaterial that is generally more conducive to hydrogen diffusionconditions. These advantages are realized with little or no sacrifice inoperating temperatures and pressures.

Particularly preferred forms of hollow fibers of this invention areshown in FIGS. 2, 3 and 4. FIG. 2 shows a photomicrograph of a crosssection of a nickel hollow fiber having a radially anisotropic internalvoid volume wall structure and a skin at the fiber's external surface.The wall structure of the fiber has an internal void volume thatincreases from the peripheral external zone to the peripheral internalzone resulting in a very open wall structure at the peripheral internalzone immediately adjacent the bore. FIG. 3 shows a photomicrograph of aportion of a cross section of a nickel-iron alloy (about 50/50 byweight) hollow fiber having a skin at the internal surface. The fiber ofFIG. 3 has an internal void volume that is lowest in the peripheralinternal zone and which increases in the peripheral external zone andhas a very open wall structure immediately adjacent the fiber's externalsurface. FIG. 4 shows a photomicrograph of a cross section of a nickelhollow fiber having a compact layer within the fiber wall which has veryopen wall structures at both the internal and external fiber surfaces.

An extremely important contribution of the present invention is theability to provide inorganic hollow fibers with varying sizes andconfigurations. The size of the fiber can be influenced by the simpleexpedient of changing spinnerets as is well known in the synthetic fiberfield. By varying the extrusion and fiber-forming conditions the wallstructure can be varied over wide ranges to provide the desired wallstructure and thickness. Furthermore, the thickness and location of acompact layer can also be provided as desired by means hereinafterdescribed. These characteristics provide those skilled in the art with aunique ability to produce fibers tailored for the application ofinterest.

These features are provided by the process of this invention which isdescribed more particularly below.

PROCESS TO PRODUCE THE FIBER Preparation of Polymer Solution ContainingInorganic Material

A mixture which comprises an inorganic material in uniformly dispersedform in a polymer solution is prepared. The polymer solution comprises afiber-forming organic polymer dissolved in a suitable solvent. Ingeneral the concentration of the organic polymer in the solution issufficient to form, when the solution contains the inorganic material,the precursor polymeric hollow fibers having a radially anisotropicinternal void volume wall structure by dry and/or wet spinningtechniques. The polymer concentration can vary over a wide range anddepends on the characteristics desired in the final hollow fiber. Themaximum concentration is, of course, limited to that where the polymersolution containing the inorganic material is not amenable to extrusionthrough a spinneret. Correspondingly, the lower limit is where thepolymeric precursor hollow fiber does not have a sufficient polymer tomaintain its wall structure. In general, the polymer concentrations willbe from about 5 to about 35% by weight of the polymer solution.Particularly preferred polymer concentrations are from about 10 to about30%, more particularly preferred 15% to 30%, by weight of the polymersolution.

The nature of the organic polymer employed in the preparation of thepolymeric precursor hollow fiber according to this invention is notcritical; for example, polyacrylonitrile, polymers of acrylonitrile withone or more other monomers polymerizable therewith such as vinylacetate, methyl methacrylate, urethanes and vinyl chloride may be used.Both addition and condensation polymers which can be cast, extruded orotherwise fabricated to provide hollow fibers by dry or wet spinningtechniques are included. Typical polymers suitable for use in theprocess of the present invention can be substituted or unsubstitutedpolymers and may be selected from polysulfones; poly(styrenes),including styrene-containing copolymers such as acrylonitrile-styrenecopolymers, styrene-butadiene copolymers and styrene-vinylbenzylhalidecopolymers; polycarbonates; cellulosic polymers, such as celluloseacetate-butyrate, cellulose propionate, ethyl cellulose, methylcellulose, nitrocellulose, etc.; polyamides and polyimides, includingaryl polyamides and aryl polyimides; polyethers; poly(arylene oxides)such as poly(phenylene oxide) and poly(xylylene oxide);poly(esteramidediisocyanate); polyurethanes; polyesters (includingpolyarylates), such as poly(ethylene terephthalate), poly(alkylmethacrylates), poly(alkyl acrylates), poly(phenylene terephthalate),etc.; polysulfides; polymers from monomers having alpha-olefinicunsaturation other than mentioned above such as poly(ethylene),poly(propylene), poly(butene-1), poly(4-methyl pentene-1), polyvinyls,e.g., poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidenechloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly(vinylesters) such as poly(vinyl acetate), and poly(vinyl propionate),poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers),poly(vinyl ketones), poly(vinyl aldehydes), such as poly(vinyl formal)and poly(vinyl butyral), poly(vinyl amines), poly(vinyl phosphates), andpoly(vinyl sulfates); polyallyls; poly(benzobenzimidazole),polyhydrazides; polyoxadiazoles; polytriazoles; poly(benzimidazole);polycarbodiimides; polyphosphazines, etc., and interpolymers, includingblock interpolymers containing repeating units from the above such asterpolymers of acrylonitrile-vinyl bromide-sodium salt ofparasulfophenylmethallyl ethers; and grafts and blends containing any ofthe foregoing. Typical substituents providing substituted polymersinclude halogens such as fluorine, chlorine and bromine; hydroxylgroups; lower alkyl groups; lower alkoxy groups; monocyclic aryl; loweracyl groups and the like.

Furthermore, since the organic polymer is to be treated to remove it insubsequent steps of the process, it should be amenable to thistreatment. For instance, a more preferred polymer would be one thatreadily decomposes and/or reacts, but not at an excessively rapid rateto effect its removal. Still further, such polymers should not formreaction products that will adversely interact with the inorganicmaterials or interface with the subsequent steps in the process.

Obviously the cheapest and most readily available polymers arepreferred. Polymers and polymers of acrylonitrile with one or moremonomers polymerizable therewith are particularly useful in the processof this invention.

The solvents used in the preparation of the polymer solution can be anynumber of those well known to those skilled in the art. For instance,such solvents as dimethylacetamide, dimethylformamide, dimethylsulfoxide, etc., are particularly useful with such polymers ofacrylonitrile. Obviously the solvent selected should be a good solventfor the organic polymer and should be amenable to the dry or wetspinning techniques contemplated in the subsequent steps of the process.

The polymer solution containing an inorganic material can be prepared bydispersing the inorganic material in the solvent followed by theaddition and dissolution of the polymer in the solvent. Any othersuitable means of preparing the polymer solution containing an inorganicmaterial is acceptable, for instance, by concurrently mixing polymer,inorganic material and solvent or by mixing the polymer and the solventfollowed by addition and dispersion of the inorganic material, etc. Itis preferred to disperse the inorganic material in the solvent prior topolymer addition.

Ambient or somewhat higher temperatures are usually quite adequate forthe preparation of the polymer solution containing an inorganicmaterial. Dependent on polymer, solvent and/or inorganic materialutilized, higher or lower temperatures may aid the preparation but arenot considered critical.

The amount of the inorganic material is inversely related to the samegeneral considerations discussed above concerning the polymerconcentration in the polymer solution. The maximum amount is limited tothat where the precursor fiber structure can not be maintained becausesufficient polymer is not present. The minimum amount is where theinorganic material particles are so widely dispersed that they do notsufficiently fuse or bond during sintering. Normal ratios, by weight, ofinorganic material to polymer will range from about 3.5 to about 15.Preferred ratios of inorganic material to polymer are from about 4 to12, more preferably from about 4.5 to 10.

The inorganic material must be uniformly dispersed as, e.g., smallparticles, throughout the polymer solution. Sufficient mixing must becarried out to achieve such a uniform dispersion. Although some amountof inorganic material may be dissolved, and this may be helpful inachieving a uniform dispersion, this is not critical to achieving theobjectives of the present invention.

The inorganic material incorporated into the polymer solution is asinterable inorganic material (this phrase includes materials from whicha sinterable material can be prepared). Such materials constitute anextraordinarily large group of materials that either are suitable assuch or that can be converted to the desired sinterable inorganicmaterial. For instance, if the desired fiber is to comprise a metal,such as nickel or its alloy, either the metal, its oxide or othercompounds that can be ultimately converted to such metals can be used.

Although the process of the present invention is particularly useful inproducing hollow fibers of metals, such as by the reduction of metaloxides to elemental metal and sintering of the metal, it may be utilizedto produce hollow fibers of any inorganic materials that are sinterable(or that can be converted to a sinterable material). Such inorganicmaterials are discussed above. For purposes of illustration, thefollowing detailed description will be limited to metal compounds whichare reducible to metals which are sinterable.

Since the reduction temperatures must, of course, be below the meltingand vaporization point of the compounds being reduced and of theelemental metal formed, the metal compounds which vaporize or sublimeexcessively at temperatures below that at which they will react withhydrogen or carbon, the metal component of which has such a lowtemperature of vaporization of sublimation (e.g., K, Na, Li, etc.), maynot be satisfactorily used in accordance with the present processwithout special considerations. (Although the use of hydrogen to providethe environment for reducing the metal compound particles to elementalmetal is a preferred embodiment of the present invention, other reducingmaterials may be employed. For example, the metal compounds andparticularly nickel and iron oxides can be reduced by partially orwholly substituting carbon monoxide for the hydrogen reducingenvironment. Obviously the constituents of the polymer and traces ofsolvent will also contribute to such a reducing environment.)

Additionally the metal compound itself is limited to those materialswherein the reaction products, other than the elemental metal, willleave the reaction zone prior to or during sintering of the hollowfiber.

The most significant metal compounds are, of course, the oxides sincethese compounds are the most plentiful; and, in fact, are the state inwhich metals are most commonly found as by-products of manufacturing andin natural ore concentrates. Other compounds which may be utilizedinclude metal halides, hydroxides, carbonates, oxalates, acetates, etc.

Particle size is an important factor for producing the desired hollowfibers regardless of the inorganic material utilized. Small particlesutilized for dispersion in the polymer solution usually range in sizefrom less than 15 microns, preferably 10 microns, most preferably 5 orless microns. Generally mixtures of such particles will range in sizedistribution from one end of the scale to the other. Obviously thesmaller particle sizes are preferred in order to obtain a more uniformdispersion. To obtain metal fibers of desired characteristics it may benecessary to use very small particles, i.e., 1 micron or less. This mayrequire particle size comminution and/or classification to achievedesired sizes.

In the application of the reduction and sintering techniques to hydrogenreducible compacted metal compounds such as metal oxides, carbides,etc., other workers have found that the resulting sintered articlesexhibit microscopic cracks and fissures and a generally poor surface.This was thought to be due to "outgassing" of vaporized or sublimedelements of reaction or the compacting aids. The cracks and fissures didnot heal during the subsequent sintering step and the poor surfacecondition persisted. This "outgassing" problem is not observed with theprocess of this invention.

A generally smaller diameter particle would be expected to intensify"outgassing" cracking and surface problems since the smaller particlesare closer together leaving less room for the evolved reaction products,i.e., gases to escape. However, it has been found that where the smallerparticles are utilized a more flaw-free essentially non-porous compactlayer can be produced. A porous characteristic is, for all intents andpurposes, essentially nonexistent where the process is applied toproduce a skin at the hollow fiber surface using particles under about 1micron in size.

A still further difficulty in using very fine metal particles relates tothe tendency of many metals to oxidize when exposed to air in smallparticle form. For example, fine iron particles (40 microns or less)tend to react exothermically when exposed to air to form iron oxideparticles. Thus, it is difficult to handle such materials while theoxide particles can be freely shipped and easily handled withoutproviding air tight protective envelopes or making special provisions toavoid spontaneous reactions. The process of this invention isparticularly amenable to use of oxides since oxide particles are oftenby-products of metal treating, and, consequently, are readily availableat low prices. For example, iron oxide particles obtained as aby-product from hydrochloric acid pickling are readily available. Othersources of iron oxide particles include dust from basic oxygenconverters, rust, mill scale, and high-grade iron ore. Nickel oxide isavailable at nominal prices.

Metal compound particles of any general shape (i.e., spherical, oblong,needles, or rods, etc.) may be employed in accordance with the presentinvention. Metal oxide particles obtained by the process of spray dryinga dissolved metal compound can provide superior hollow fibers.

Accurate particle size determinations of small particles are difficultto obtain, particularly where the particle size includes particles lessthan 10 microns in diameter (or smallest dimension). Such determinationsare most difficult where the particles are of non-uniform shape. Forexample, many of the particles are likely to be of a relativelyelongated configuration so that it is difficult to determine thesmallest dimension of the particle. Elongated particles will not passthrough a screen having a mesh that is designed to accommodate arelatively symmetrically shaped particle of equivalent mass. As a resultparticle size and particle size distribution measurements vary to aconsiderable degree for a given material between the known methods andprocedures for making such determinations.

Relatively accurate small particle size determinations may be madethrough the use of the Coulter counter procedure. In this procedure theparticles are suspended in an electrically conductive liquid and arecaused to flow through a small orifice. A current is caused to flowthrough the orifice by means of two immersed electrodes, one on eachside of the orifice. As the particles flow through the orifice, thechange of electrical resistance between the electrodes is measured todetermine particle size. Thus, the measure is primarily interpreted onparticle mass and is not affected by shape.

The process of the present invention, when using metal compounds, takesadvantage of the "active" state of the metal after reduction of themetal compound particles and prior to sintering. Metal particles tend toacquire a thin oxide coating or film and in fact nearly all metalpowders of fine particle size must acquire or be provided with such afilm to prevent rapid oxidation or defeat the pyrophoric nature of suchmaterials. Such a film renders the particles "passive" so that they maybe handled in ordinary atmosphere. However, such a film is difficult toreduce and retards sintering. When metal compound particles are reducedin accordance with the process of the present invention to elementalmetal and such metals are sintered subsequent to reduction without beingexposed to an oxidizing environment, hollow fibers of this inventionhaving excellent properties are obtained.

Metal alloys can be provided as the inorganic material of the fiber ofthis invention by the simple expedient of mixing particles of metalcompounds, e.g., metal oxides, and dispersing this mixture in thepolymer solution. Such alloys can provide useful characteristics ofstrength, diffusivity and chemical resistivity. Exemplary of such alloysare those formed using nickel and iron oxides.

Another acceptable procedure for making metal hollow fibers by thepractice of the process of the present invention is to incorporate metalparticles with the particulate metal compounds. Preferably the metalparticles will be blended with the metal compounds prior to dispersionin the polymer solution. Reducing and sintering may be accomplished atthe usual temperatures and in the presence of the usual atmospheres (inaccordance with the process of the present invention). The sinteringtemperature may be high enough to effect diffusion of the elementalmetal into the reduced base metal to effect alloying. Consequently, itmay be necessary or desirable to employ a somewhat higher sinteringtemperature where the elemental metal has a low diffusion rate. If thesintering temperature of the elemental metal (or temperature at whichdiffusion of the elemental metal into the base metal will occur) ishigher than the melting point of the base metal then alloying may not beaccomplished. However, in the latter eventuality the elemental metal orits oxide may dispersion strengthen the base metal.

An additional use of metal particles is to reduce shrinkage of thesintered fiber. In any sintering process, the metal article shrinks inits outer dimensions due to the elimination of the void spaces betweenthe particles when the particles fuse to form a solid mass. When theinorganic material comprises metal compounds such as metal oxides thatare first reduced and then sintered in accordance with the method of thepresent invention such shrinkage is accentuated due to the fact that thereduced particles are smaller than the metal compound particles and thusshould provide greater void spaces between particles. Such shrinkage canbe reduced or minimized by adding elemental metal particles to the metalcompound particles for incorporation in the polymer solutions. Forexample, it may be desirable to add up to 50%, by weight, nickelparticles to nickel oxide particles to reduce shrinkage of the resultanthollow fiber. The particle size of the elemental metal particles willpreferably be very small since such dispersed particles will diffuseinto a matrix metal quickly and evenly.

Further, by including with the metal compound a proportion of dispersed,non-reducible (or diffusible) materials of controlled particle size, itis possible to effect a dispersion strengthened sintered fiber. Theparticles may consist of elemental metals that sinter at a highertemperature than the sintered material of the fiber.

As mentioned above, the sinterable inorganic material can be a materialthat comprises the fiber material without chemical modification or amaterial that is converted to a desired form by chemical modification.As extensively discussed above, metal compounds particularly metaloxides to be reduced to elemental metals, are illustrative of the lattermaterials. If metal fibers are desired these oxides require reduction tothe elemental metal prior to or during sintering. Other materials thatare amenable to the process of the present invention are those that mayrequire oxidation or both oxidation and reduction to form the materialcomprising the final hollow fiber. Although these procedures will not bediscussed in the detail provided for metal compounds, those materialswhich may be oxidized prior to sintering, such as aluminum, are alsouseful with the process of this invention. Other inorganic materialswhich can be provided by simultaneous oxidation and reduction are alsouseful in the process of this invention. Illustrative of these materialsis the simultaneous oxidation and reduction of aluminum or titanium andiron oxide or nickel oxide. The following materials illustrate thosematerials which can form the final fibers without chemical modification(i.e., without reduction and/or oxidation), are metals, ceramics such asalumina, β-alumina, glass, mullite, silica, etc.

The polymer solution containing an inorganic material can also containother additives to assist in this and subsequent steps in the process,particularly for instance, in the extrusion and fiber-forming steps.Wetting agents such as sorbitan monopalmitate, etc. are useful to wetthe inorganic material by the solvent of the polymer solution.Plasticizers such as N, N-dimethyl lauramide, etc. are useful to providepolymeric precursor fiber flexibility.

Extrusion of Polymer Solution Containing Inorganic Material

In making hollow fibers of the present invention, a wide variety ofextrusion conditions may be employed. As previously discussed, theweight percent polymer in the solution may vary widely but is sufficientto provide a hollow fiber under the extrusion and fiber-formingconditions. If the inorganic material, polymer and/or solvent containcontaminants, such as water, particulates, etc., the amount ofcontaminants should be sufficiently low to permit extrusion and/or notinterfere with or adversely affect subsequent steps in the process orthe final fiber. If necessary, contaminants can be removed from thepolymer solution by filtration procedures. Obviously filtration must beappropriate to remove contaminant particles while passing the particlesof inorganic material. Such filtration may also remove particles ofinorganic material which are above the desired particle size. Thepresence of excessive amounts of gas in the polymer solution containinginorganic material may result in the formation of large voids andundesirable formation of porosity in the polymeric precursor hollowfiber. Accordingly, degassing procedures are also appropriate. Suchdegassing and/or filtration procedures can be carried out immediatelyafter or during preparation of the polymer solution containing aninorganic material or can be carried out immediately prior to or duringthe extrusion step.

The size of the hollow fiber spinnerets will vary with the desiredinside and outside diameters of the resultant polymeric precursor hollowfiber. The spinnerets may also vary in shape, i.e., hexagonal, oblong,star, etc. The spinnerets are generally circular in shape and may haveouter diameters of, for instance, about 75 to about 6000 microns withcenter pin O.D. of about 50 to about 5900 microns with an injectioncapillary within the center pin. The diameter of injection capillary mayvary within the limits established by the pin. The polymer solutioncontaining the inorganic material is frequently maintained under asubstantially inert atmosphere to prevent contamination and/orcoagulation of the polymer prior to extrusion and to avoid undue firerisks with volatile and flammable solvents. A convenient atmosphere isdry nitrogen.

The temperature preparatory for extrusion of the polymer solutioncontaining inorganic material can vary over a wide range. In general thetemperature is sufficient to prevent undesirable coagulation orprecipitation prior to extrusion. The temperature generally can rangefrom about 15° C. to about 100° C. preferably from about 20° C. to about75° C.

The pressure to accomplish the extrusion is normally those within theranges understood by those skilled in the fiber spinning arts. Thepressure depends on, for instance, the desired extrusion rates, thespinneret orifice size and the viscosity of the polymer solutioncontaining the inorganic material. Of particular note is the fact thatrelatively low pressures can be utilized with the process of the presentinvention. This contrasts with compaction procedures which often requirehundreds of atmospheres of pressure to provide compacted and sinteredarticles. The pressures useful with the present invention normally rangefrom about 1 atmosphere up to about 5 atmospheres or higher.

Obviously the fibers can be extruded through a plurality of spinnerets.This will enable the concurrent formation of multiple fibers while, forinstance, using the same coagulating bath. The use of a plurality ofspinnerets can also enable the twisting together of the precursor fibersduring or subsequent to formation. This provides a particularly uniqueability to provide multiple fiber cords that are particularly suited forgood fluid distribution to the outer fiber wall diffusion surfaces whenjoined in bundles of many fibers. Such twisted fibers are particularlyuseful in achieving desirable packing factors when assembling the cordsin a bundle and result in excellent distribution of fluids therein. Thiscontrasts with bundles of relatively straight fibers which generally maynot exhibit such desirable fluid distribution patterns.

Formation of the Polymeric Precursor Hollow Fiber

In general, fiber-forming spinning techniques are known to those skilledin the synthetic fiber-forming industries. These skills can beadvantageously applied to the fiber-forming step of the process of thisinvention. Likewise, procedures have been developed to form polymerichollow fibers having radially anisotropic internal void volume wallstructures. Such procedures can also be readily adapted to thefiber-forming step of the instant invention. These latter procedures areexemplified by the following patents which are hereby incorporated byreference: U.S. Pat. Nos. 3,674,628, 3,724,672, 3,884,754 and 4,055,696.The fiber-forming step may be conducted using wet or dry spinningtechniques, i.e., the spinneret may be in or removed from thecoagulating bath. The wet technique is often preferred and may be usedfor the sake of convenience.

The coagulation can be effected by bringing the fiber which is beingformed into contact with a coagulating bath. In the case of theperipheral external zone it suffices to pass the fiber which is beingformed into the coagulating bath. The peripheral internal zone can besubjected to coagulation by injecting a fluid (which coagulates thepolymer in the polymer solution) into the bore of the fiber beingformed. The fluid may comprise, e.g., air, isopropanol, water, or thelike. The size of the polymeric precursor hollow fiber can be increasedby an increased flow of the fluid injected into the bore.

Any essentially non-solvent for the polymer can be employed as thecoagulating agent in the coagulating bath. The coagulating agent isnormally miscible with the solvent of the polymer solution. The natureof the coagulating agent selected depends on the solvents used for thepolymer and the choice depends on criteria known in the field of fiberspinning. By a "powerful coagulating agent" is meant a medium in whichthe polymer will rapidly precipitate. By a "mild coagulating agent" ismeant a medium in which the polymer will precipitate slowly.Conveniently, water is employed as the primary coagulating agent in thecoagulating bath. Other coagulating agents are ethylene glycol,polyethylene glycol, propylene glycol, methanol, ethanol and propanol,etc. The residence time for the extruded fiber in the coagulating bathis at least sufficient to ensure reasonable solidification of the fiber.The peripheral external zone is formed due to interaction with thecoagulating agent and/or cooling. (Cooling may also be achieved bybringing the extruded polymer solution containing inorganic materialinto contact with a gas at a temperature below the gelling temperatureof the polymer solution. Where gelling is accomplished in this manner,the cooling gas can be subjected to a relatively rapid translatorymovement which can be oriented in a direction parallel to that of thehollow fiber. This gas may additionally be charged with water vapor orthe vapor of some other non-solvent). The setting of the peripheralinternal zone can be achieved in a similar manner by interaction with acoagulating agent in the injected fluid and/or by cooling due to thetemperature of the injected fluid. Where gelling is also accomplished inthe coagulating bath the bath may, in addition to its gelling effect,also impart a coagulating effect.

The temperature of the coagulating bath may also vary widely, e.g., from-15° to 95° C. or more, and is most often about 1° to 35° C., say, about2° to 25° C. The temperature of the fluid injected into the bore can befrom about -15° to about 95° C. preferably about 1° to about 35° C.

In forming the polymeric precursor hollow fibers of this invention theradially anisotropic internal void volume wall structure can be realizedby using different temperatures and compositions of the coagulating bathand the fluid injected into the bore. For instance, to achieve highinternal void volume the coagulating agent in either the coagulatingbath (for the peripheral external zone) or the fluid injected into thebore (for the peripheral internal zone) should be a powerful coagulatingagent or should have a higher concentration of a coagulating agent. Toachieve lower internal void volumes mild coagulating agents can beutilized. Different temperatures can also effect rate of coagulation.

The wall structure can also be varied by, for instance, pumping rate fora given take-up speed, the amount of fluid injected into the bore, thedegree of stretching, etc. A compact layer at the external surface ofthe fiber wall can be obtained by, for instance, using a very mildcoagulating agent (or low concentration) in the coagulating bath. Acompact layer at the internal surface of the fiber wall can be obtainedby, for instance, using a very mild coagulating agent (or lowconcentration) in the fluid injected into the bore. A compact layerwithin the fiber wall can be obtained by, for instance, using a verypowerful coagulating agent in both the coagulating bath and fluidinjected into bore.

The process of this invention provides particularly desirableanisotropic hollow fibers that have an essentially non-porous compactlayer. Such layers are present as internal and/or external skins or arewithin the fiber wall. The essentially non-porous compact layer canusually be achieved by the procedures described above.

After coagulating the fiber it may be washed to remove solvent by, forinstance, washing with the coagulating bath solution or with othernon-solvents that are miscible with the solvent of the polymer solution.The precursor hollow fiber may also be stored in a water or other liquidbath.

The extrusion and fiber-forming conditions are preferably such that thefiber is not unduly stretched. Although not necessary, stretching can beused, say about 1 to about 5 fold. Frequently, extrusion andfiber-forming speeds are within the range of about 5 to 100 meters perminute although higher speeds can be employed providing the fiber is notunduly stretched and sufficient residence time is provided in thecoagulating bath. Stretching generally strengthens the polymericprecursor hollow fiber. Stretching also allows increased linearproductivity and smaller fiber diameters with a given spinneret.

An annealing procedure may also be carried out to toughen the polymericprecursor hollow fiber. Both the stretching and annealing procedures canbe conducted by, for instance, passing the fiber through boiling water.

Another important consideration, but not a limitation, on hollow fiberwall structure is the presence of a compact layer having a minimum of"flaws". (This term when used in the present context refers toimperfections in the compact layer through which, under normal operatingconditions, the passage of both desirable and undesirable fluids isallowed without the desired discrimination). The upper limit on flaws isa matter of compromise in each system for a number of reasons. Somesystems by reason of economics require a very high selectivity whileothers may require only moderate selectivity to be competitive withother separation techniques. Thus, generally, while precautions inhollow fiber production and handling should be taken to minimize flaws,the acceptable number and sizes of flaws will vary depending on theapplication of the fiber.

The precursor hollow fibers of polymer laden with an inorganic materialcan be subjected to the subsequent steps in the process or can be takenup and stored in precursor monofilament form, or as twisted cords, on,for instance, bobbins. The precursor fibers are flexible and have areasonable degree of strength and can therefore be handled without undueconcern for damage.

After obtaining the precursor fiber by the process of this invention,drying can be carried out in a known manner. The fibers are generally,but not necessarily, dried prior to treatment to remove the organicpolymer. The drying may be conducted at about 0° to 90° C., convenientlyabout room temperature, e.g., about 15° to 35° C., and at about 5 to 95,conveniently about 40 to 60, percent relative humidity.

The precursor fiber comprises the polymer in minor amount acting as thecontinuous phase carrier for the inorganic material which is uniformlydispersed throughout the polymer. Generally, the polymer is present inthe precursor fiber in concentrations substantially less than 50% andoften as low as 25%, 15 or as low as about 5% by weight. The majorcomponent in the precursor fiber being, of course, the inorganicmaterial. Other materials may be present in the precursor fiber butgenerally only in small amounts.

FIG. 1 shows a polymeric precursor hollow fiber prepared by theforegoing procedure.

Treatment to Remove Organic Polymer

After formation of the polymeric precursor hollow fibers laden withinorganic material the fiber can preferably be dried or dried and storedas discussed above, or transferred directly to a treatment to remove theorganic polymer from the fiber. This can be accomplished by heating todecompose and/or react the organic polymer. This may be accomplished inan inert or reducing atmosphere to aid in reduction of the inorganicmaterial, although this is not always necessary. As mentioned above, thereaction products formed from the organic polymer may serve to enhancethe other steps of the process. For instance, the hydrogen and carbonpresent in the polymer serve as an excellent source of a reducingenvironment. This environment helps to reduce metal compounds, e.g.,oxides, to the elemental metal.

The fiber containing inorganic material may, optionally, be subjected toreduction and/or oxidation. (It is, of course, recognized that neitherreduction nor oxidation may be necessary if the inorganic materialdispersed into the polymer solution is in the chemical form desired forsintering.) Preferably an appropriate atmosphere will be provided justprior to the fiber being subjected to the reduction and/or oxidationtemperature. For instance, with reduction, this may be accomplished bycontinuously passing the polymeric precursor hollow fiber laden with areducible inorganic material through a commercially available oven. Anatmosphere comprising, for instance, hydrogen may be caused to flowcountercurrently and in contact therewith. As the fiber first contactsthe heat of the oven, the remaining volatile components will outgas. Asthe temperature approaches reducing temperatures, the reducibleinorganic material, for instance, metal compounds, are reduced, forinstance, to elemental metal, and the reaction products outgas.

For the purposes of the preent invention and this specification, it willbe understood that the temperature range at which polymer removal andreduction and/or oxidation will occur and the sintering temperatures mayoverlap to some extent. In other words, some sintering may occur at thetemperatures at which polymer removal and reduction and/or oxidation iscarried out, although it is preferable that the temperature be such thatreduction takes place immediately preceding sintering. The preferredtemperatures at which reducible inorganic materials, i.e., metalcompounds, will reduce are well-known to those skilled in the art ortheir determination is well within the skill of those of ordinarycompetency.

The preferred reducing environment may be provided by any atmospherewhich provides a source of hydrogen. For example, such an atmosphere maycomprise hydrogen, cracked hydrocarbons, dissociated ammonia,combinations of each, combinations of one or more of such gases andother gases or vapors which will not materially interfere with thereduction reaction. The reaction products from the decomposition and/oroxidizing of the polymer are valuable aids in providing the reducingatmosphere.

Solid reducing materials, carbon for example, may be employed incombination with the hydrogen yielding gas only where the reactants(e.g., CO and CO₂) appropriately "outgas" and will not leave residualelements in the sintered fiber that will interfere with the desiredfiber properties. For example, carbon may be a desired addition to theoxide powder. Carbon may also be employed where the ultimate product iscarbide-containing, e.g., a steel composition where the residual carbonis a necessary element for the final fiber.

Oxidation of the inorganic material can be conducted at the appropriatetemperatures under suitable pressures and atmospheres. Air is thepreferred atmosphere. The oxidation temperatures are generallywell-known or readily ascertainable. Simultaneous oxidation andreduction can occur, say, for instance, in the formation of cermets.

The resulting fiber comprising a sinterable inorganic material may thenbe conducted directly into a sintering zone.

Sintering to Form to Inorganic Fiber

The term "sintering" is meant to include an agglomeration by fusion andbonding of the sinterable inorganic material to at least that point atwhich the particulate material forms a monolithic structure. Sinteringshould provide a fiber having substantial strength as compared to afiber which has undergone the previous steps and has not been sintered.The sintering must be conducted under conditions that assure that thevalence state desired is achieved or maintained under sufficienttemperatures and times to allow the fusion and bonding to occur.

In the production of the hollow fibers of this invention there arelittle or no limitations on the heating rate for sintering. Forinstance, the sintering of a nickel-iron alloy fiber can be from about950° C. to about 1200° C. for from 15 to 5 minutes, respectively. Anickel-iron alloy fiber produced under these conditions is excellent. Ingeneral, similar to the reduction and oxidation temperatures, thepreferred sintering temperatures of the inorganic materials arewell-known or readily ascertainable.

During the organic polymer removal, optional reduction and/or oxidationof the inorganic material and sintering steps, suitable conditions mustbe maintained to avoid damage or destruction to the fiber wall structureand integrity. A shrinkage ratio (final fiber to precursor fiber) offrom about 0.2 to about 0.9 can be expected, usually 0.3 to 0.6. Thatis, the precursor hollow fiber is often transformed to the final hollowfiber with substantial size reduction. This is expected during theseprocess steps. For instance, the fiber is substantially reduced inlength and the fiber outer diameter, wall structure and compact layer,although remaining in relative relationships, are also reduced in size.During these steps means must be provided to handle the fiber as itshrinks. Particularly critical is the point immediately prior tosintering where the fiber is fairly fragile. At this point, particularcare must be taken to provide means to afford such shrinkage withoutdamage to the fiber. For instance, if the fiber is allowed to adhere toa conveying surface at this point it may break as it shrinks. One methodof handling the fiber at this point is to feed a precursor fiber or acord of precursor fiber, which may be pretreated, to provide betterhandling characteristics, into the furnace by means of a conveyor beltwhich is fabricated of material which does not adhere to the fiber underthe operating conditions of the furnace. This conveyor belt can betransporting the fiber at the speed of the final fiber as it exits thefurnace. The precursor fiber feed speed is faster than the final fiberspeed. The precursor feed speed can be adjusted to account for theshrinkage that occurs.

Those fibers having a compact layer can be treated to obtain a porouscompact layer by, for instance, treating the compact layer with a fluidthat has some interaction with the material of the compact layer toproduce a porous compact layer. For instance, a polymeric precursorfiber containing nickel oxide and a compact layer can result in auniformly porous surface by introducing ammonia gas in the atmosphere inthe furnace. The photomicrograph shown in FIG. 6 illustrates such auniformly porous compact layer.

An alternate means to obtain a porous compact layer is to introduce arelatively small amount of fine particulate material which does notparticipate in the sintering or participates in the sintering to alesser degree. Incorporation of such fine particulate materials in thepolymer solution containing an inorganic material during its preparationhas resulted in a porous compact layer in the final inorganic fiber.

A particularly important feature of the process of this invention is theability to produce fibers having essentially non-porous compact layerswith ease. This feature is surprising since the polymer of the polymericprecursor fiber is the continuous phase which is removed as discussedabove. It has been found that, although the polymer is removed from thecompact layer of a precursor fiber, the final fiber, after sintering, isusually essentially non-porous. Although it might be expected thatshrinkage and reduction of interstices between particles of inorganicmaterials might occur when the inorganic material undergoes reduction,oxidation and/or sintering, the formation of a compact layer that isessentially non-porous, i.e., allows passage of fluids, e.g., gas,essentially only by diffusion, is both desirable and unexpected. Thisphenomena appears to occur throughout the fiber wall structure whereeverpolymer is removed. It has been observed particularly when using metalcompounds, e.g., oxides, to convert to elemental metal.

The essentially inorganic, monolithic hollow fiber having a radiallyanisotropic internal void volume wall structure resulting from theforegoing process is strong compared to precursor fiber and fibers fromthe intervening steps. The final fibers may be flexible enough to bestored on bobbins but are not as flexible as the precursor fibers. Thefinal fibers can be cut into desired lengths for assembly into, forinstance, bundles having a multiplicity of fibers (which may also be incords of twisted fibers). Usual lengths range from about 0.2 to about 10meters, preferably about 1 to about 5 meters. The size of the bundles isdependent on the application intended but can generally range from about0.5 to about 25 cm in diameter. Likewise, the devices utilizing thefiber bundles can contain multiple bundles. Procedures for constructingsuch devices are known to those skilled in the art. See, for instance,U.S. Pat. No. 2,961,062 which is hereby incorporated by reference.

Metal Radially Anisotropic Internal Void Volume Hollow Fiber

A metal hollow fiber and process to produce it that are preferredembodiments of the present invention are described below. This metalanisotropic hollow fiber has an essentially non-porous thin skin at itsexternal surface.

The organic polymer solution can comprise, for instance, anacrylonitrile homopolymer or polymers of acrylonitrile with one or moremonomers polymerizable therewith dissolved in an organic solvent such asdimethylacetamide, dimethylformamide, etc. Generally the concentrationof polymer in the solution can be from about 5 to about 35 preferablyfrom about 10 to about 30% by weight of the polymer solution. The metalscomprising the fibers can preferably be provided by, for instance,dispersing small particles of metal compounds, e.g., oxides of themetal, into the polymer solution. Preferred metals are those metalalloys, such as nickel-iron alloys, which can be obtained by mixingsmall particles of oxides of the metals desired, for instance nickel andiron oxides. Generally, any such metal oxide mixture may contain apredominant amount of one metal oxide, e.g., nickel, at say from about65 to about 99% by weight of the metal oxide with say, from about 35 to1% of another metal oxide, e.g., iron oxide. The small particles of themetal compound are preferably mixed with the solvent prior to additionof the polymer. This may be of particular advantage if a particle sizereduction is contemplated during such mixing. The amount of metalcompound can generally range from a weight ratio of metal compound topolymer of about 3.5 to about 15, preferably from about 4 to about 12,more preferably 4.5 to 10. The mixture might also contain small amountsof other materials. For instance, wetting agents may be particularlyuseful in achieving the desired uniform dispersion of the metal compoundthroughout the polymer solution. The temperature utilized during themixing is not particularly significant except to the extent that asufficiently high temperature should be maintained to form the desiredpolymer solution containing a uniform dispersion of the metal compound.

During or subsequent to the formation of the polymer solution containingthe metal compound(s) it is preferred to utilize particulate removal,e.g., filtering and/or degassing procedures to remove undesirable solidparticles (which may also include excessive sized metal compoundparticles) and/or undesirable gases.

The polymer solution containing the metal compound can then be extrudedthrough a hollow fiber spinneret having, for instance, an outer diameterof from about 75 to about 6000 microns, preferably from about 200 toabout 1000 microns and center pin O.D. of from about 50 to about 5900,preferably from about 50 to about 900 microns. The center pin can alsohave an injection capillary.

The fiber being extruded from the spinneret orifice is then, preferably,immediately contacted (as in wet spinning) with a coagulating bath. Thecoagulating bath should contain a non-solvent, e.g., water, for thepolymer and, usually, may also contain the solvent of the polymersolution. When homopolymers or polymers of acrylonitrile with monomerspolymerizable therewith are used as the polymer it has been found to beparticularly advantageous to use water as a coagulating agent both inthe coagulating bath and the fluid injected into the bore of the fiberbeing extruded. The coagulating agent concentration in the coagulatingbath is dependent on the desired rate of coagulation. The rate ofcoagulation is also temperature dependent. It is generally necessary tohave a coagulating agent, for instance water, concentration of fromabout 20 to about 100%, preferably about 35 to about 100%, by volume, ofthe coagulating bath. A temperature of the coagulating bath below thetemperature of the mixture being extruded is often advantageous. Theconcentration of the coagulating agent (which may be the same ordifferent from the coagulating agent in the coagulating bath) in thefluid injected into the bore of the extruded fiber is also dependent onthe fiber characteristics desired. Usually a higher concentration of apowerful coagulating agent in the injection fluid is acceptable where ahigher internal void volume in the peripheral internal zone is desired.Often water is quite acceptable as the injection fluid.

The precursor hollow fiber can then be passed from the coagulating bathto a stretching procedure, say from about 1 to 5 fold in a suitablemedium, for instance, boiling water. (A washing procedure may beutilized after the coagulating bath in lieu of longer residence times inthe bath.) The fiber can also be subjected to a relaxing (annealing)procedure which also can be carried out, for instance, in boiling water.The relaxing may be from about 0.6 to about 0.9 ratio. Neither thestretching or relaxing procedures are critical although they do providea stronger and tougher precursor fiber.

The resulting precursor hollow fiber comprises the polymer laden withthe metal compound(s) and having a radially anisotropic internal voidvolume wall structure. It preferably has a compact layer, e.g., skin, atits external surface. The polymer concentration in the precursor fibercan generally be relatively low, say from about 25% to 5%, preferablyfrom about 15% to about 5% by weight of the precursor fiber with theother major component being the metal compound(s). There may also besmall amounts of other materials present, i.e., traces of othersolvents, coagulating agents, wetting agents and minor contaminants,etc.

The precursor fiber can be dried at this stage, and this can usually beaccomplished by air drying. The production rate of the precursor fiberis generally from about 5 to about 100, preferably 35 to 65, meters perminute.

It is also a preferred procedure to twist a plurality, i.e., 2 or more,of precursor hollow fibers into a cord which will maintain thisconfiguration after the subsequent steps to convert the precursor fiberto a metal fiber. Such cords of metal fibers are particularly useful toprovide desired distribution patterns and packing factors when the cordsare used in bundles for separation devices. Handling such fibers in cordform is also useful to improve production rates.

The precursor hollow fiber is preferably subjected to those temperaturesand atmospheres that will decompose and/or react the polymer, reduce themetal compound to elemental metal and sinter the resultant metalparticles to form the final fiber. The reducing environment utilized maybe provided, at least in part, by the reaction products from the polymeras it decomposes or oxidizes. (The metal compounds, e.g., oxides, acthere as oxidizing reactants as they reduce.) Other inert or reducinggases, such as nitrogen, hydrogen and/or carbon monoxide, can beintroduced, preferably in countercurrent fashion, to maintain thedesired reducing atmospheres.

The metal hollow fiber can usually be taken up on a bobbin for storagefor future use or can be directed to other procedures to incorporate thefibers into devices for their use. Particularly preferred fibers arethose of nickel alloy having an outer diameter of up to about 600microns, preferably up to about 500 microns, an inner diameter of fromabout 100 to about 400 microns and a compact layer having a thickness offrom 4 to 50 microns.

Inorganic Anisotropic Hollow Fiber Applications

As previously noted the inorganic anisotropic hollow fibers of thepresent invention have numerous fields of application. Since theinorganic material comprising the fiber can be selected from a verylarge group of materials the fibers are equally diverse in their fieldsof application. This selection being limited only by the operatingenvironment anticipated for the fiber. To a significant extent theadvantages provided derive from the large surface areas available (bothwithin the wall structure and at the internal and external surfaces) andready access to these surface areas. Illustrative of such fields ofapplication are fluid separations by membrane, filtration, gas sparging,fuel cells, and batteries. Other uses will be readily apparent to thoseskilled in the art.

A particularly advantageous field of application is fluid separation bymembrane. The fibers are useful in this field both with and without acompact layer with the compact layer being either porous or essentiallynon-porous. For instance, there are numerous fluid separations that canutilize the fibers of this invention that do not have a compact layer orthat have a compact layer that is porous. These types of fibers can actas excellent supports for both inorganic and polymeric separationmembranes.

The fibers useful as supports for inorganic or polymeric membranes mayhave uniformly porous compact layers at the surface contacting themembrane. Porosity can also be provided in the fibers of this inventionby providing aniostropic internal void volume wall structures without acompact layer having small pores at the support contacting surface.Fibers with a porous compact layer are preferred as supports for suchmembranes.

The inorganic membranes to be supported by these fibers comprise metals,or other inorganic materials suitable for fluid separation by membraneprocesses. For instance, palladium, platinum and silver are excellenthydrogen diffusible metal membranes that can be supported by thesefibers. Various methods of applying such materials are known to thoseskilled in the art.

The polymeric membranes to be supported by these fibers are comprised ofa wide range of polymeric materials such as polysulfones, celluloseacetates, etc. Those skilled in the art are also well versed in suchpolymers and methods of application to the fiber surface.

Obviously the inorganic material of the fiber should be of a nature thatis satisfactory for use with the pressures, temperatures, and chemicalenvironments in which they are to be employed as supports. Theseenvironments can normally be substantially more severe than those wherepolymeric supports are used.

Such fibers can also be used in filtration processes. For instance, theycan be readily adapted for use to remove particulate matter from bothliquid and gaseous streams. Furthermore, these fibers can also beprovided with porosities suitable for use in ultrafiltration processes.

In general, these fibers can be employed advantageously whenever a largesurface area is desired and a variation of void volume is desired as afluid traverses from one side to the other. For instance, these fiberscan be used as a means of providing gas sparging, i.e., dispersion ofextremely fine gas bubbles into liquids. Another and similar applicationis the use of these fibers as porous electrodes for fuel cells. Suchfibers can be provided so that the gas side of the electrode has largevoid volumes with the electrolyte side having extremely fine voidvolumes. Such porous electrodes are particularly suitable for use inhydrogen/oxygen fuel cells. Fibers with compact layers having uniformlyporous surfaces are particularly useful in such applications.

The fibers of this invention that have an essentially non-porous compactlayer are particularly useful in gas diffusion processes. For instance,the fibers comprising hydrogen diffusible metals provide excellenthydrogen diffusion barriers which are useful in hydrogen purification,equilibrium reactions, fuel cells as the fuel electrodes, etc. Processesusing the fibers of this invention for gas diffusion, particularly thosehaving the compact layer as a skin on an internal or external surface,are preferred embodiments. Such diffusion processes are substantiallyimproved by utilizing the fibers of this invention that have anessentially non-porous compact layer. Although metals are the preferredinorganic material for use in such processes, other inorganic materialscan be equally useful in such processes. Particularly preferredprocesses are those involving hydrogen diffusion.

The effective separation of gases is substantially improved with the gasdiffusible hollow fibers of this invention over those obtainable withpolymeric hollow membranes. The fibers of this invention can use cheapermaterials, e.g., nickel, in lieu of expensive noble metals, e.g.,palladium-silver.

By employing the fibers of the present invention in gas diffusionprocesses, unique advantages in addition to those previously noted areobtained. Thus, extremely pure gas streams are obtained which can bedirectly employed, for instance, as fuel or feed, in further chemicalprocessing. Other advantages will be described in more detail below. Forinstance, as previously mentioned a particularly useful process thatutilizes the hollow fibers having an essentially non-porous compactlayer of this invention are those involving hydrogen diffusion. Hydrogendiffusion devices usually use bundles of fibers which then compriselarge surface area diffusion cells useful to selectively separate thehydrogen from hydrogen containing gaseous mixtures at a high rate. Suchcells can be prepared by fixedly securing longitudinally in a bundle amultiplicity of anisotropic hollow fibers of this invention having anessentially non-porous compact layer comprised of a hydrogen diffusiblemetal.

The hollow fibers (or cords containing a plurality of twisted fibers)may be cut to a relatively short length so that the pressure drop of thegas flowing through the device is minimized and a high diffusion ratecan be maintained. A length of about 0.2 to 10 meters provides goodresults. The fibers are gathered into a bundle. The fiber ends areusually sealed. A relatively tight fitting retaining sleeve of anysuitable metal is placed around the bundle of fibers at one end andmolten metal is introduced into the voids between the fibers and sleeve.The molten metal disperses between the exterior walls of the fibers andbetween the interior wall of the retaining sleeve and the exterior wallsof the peripheral fibers. Upon cooling, the molten metal solidifies,following which a portion of the bundle and sleeve is cut transverselyof the bundle at a point intermediate the height of the solidified metalsealant whereby the bores can be easily opened by, for instance,polishing and/or other treatments, while the fibers remain sealed toeach other and to the retaining sleeve. The bore openings of the fibersare placed in communication with a stainless steel or other suitableconduit for collecting the hydrogen and the fiber are manifolded to theconduit by sealing the sleeve to the conduit by any suitable couplingmeans. FIG. 5 shows an end view of a small bundle of fibers sealedtogether. The fibers show the anisotropic internal void volume wallstructure characteristic of the fibers of the present invention.

In practicing the instant invention it may or may not be desired thatthe bores of the fibers in the bundle be open at both ends. If sodesired, the sealing and cutting operations previously described can beapplied to the bundle at the opposite end. If not, then the bores of theindividual fibers of the bundle are permitted to remain closed at theiropposite ends. In making diffusion cells having fibers whose bores areclosed at one end, it may also be desired to seal the fibers together atthis end. If so, the sealing operation described above can be repeatedat this end but the bores of the fibers are not cut open.

The hydrogen diffusion device of the instant invention may be utilizedin processes where it is desired to separate hydrogen from other gases,to remove hydrogen to shift an equilibrium reaction or to simply providehydrogen of high purity. With reference to FIG. 7, illustrative of sucha device, within casing 21 is positioned a multiplicity of hydrogendiffusible hollow fibers of this invention, say about 2000 to 3000,arranged in a bundle generally designated by the numeral 22. One end ofthe bundle is embedded in header 23 such that the bores of the hollowfiber are in communication through the header. The header is positionedin casing 21 such that essentially the only fluid communication throughthe header is through the bores of the hollow fibers. The opposite endsof the hollow fibers are sealed in end seal 24. A gaseous mixturecontaining hydrogen, at an appropriate elevated temperature, enters thecasing 21 through feed port 25 disperses within bundle 22 and passes tocasing exit port 26 positioned at the opposite end of the casing.Hydrogen diffuses through the fiber walls to the bores of the hollowfibers and passes, via the bores, through header 23. The hydrogen exitscasing 21 through hydrogen exit port 27. While FIG. 7 depicts a hollowfiber hydrogen diffusion device in which only one end of the hollowfiber is open, it is apparent that both ends of the hollow fibers can beopen.

Particularly preferred processes utilizing the metal anisotropic hollowfibers of the present invention that have an essentially non-porouscompact layer are those which require high temperatures and pressures toproduce hydrogen. For instance, natural gas (methane), otherhydrocarbons or methanol-water reforming processes to generate hydrogenare particularly amenable to employing such fibers. Methanol-waterreforming is of particular interest. Nickel and nickel alloys areparticularly desirable metals for the hollow fibers useful in suchprocesses. Most preferred processes are those where water vapor ispresent. Apparently, the presence of water abates the deposition ofcarbon on the metal surface. Thus, the presence of water can avoid thedeleterious effects to the nickel or nickel alloy fiber surface whichmight otherwise be observed with little or no concentrations of water.Ammonia dissociation is another process amenable to use with such fibersto produce relatively pure hydrogen via hydrogen diffusion. Ammonia,however, appears to deteriorate nickel or some nickel alloy fibers tosome extent requiring separate dissociation.

A particularly advantageous feature of the fibers of this invention istheir ability to participate in different chemical reactions occurringon opposite sides of the fiber walls. This may be of advantage, forinstance, where an endothermic reaction is occurring on one side of thewall to produce hydrogen which can diffuse through the fiber wallessentially non-porous compact layer. The heat to carry out such areaction and maintain an appropriate temperature could be provided by,for instance, providing an oxygen containing gas, e.g., air, on theopposite side to cause an exothermic oxidation reaction with thehydrogen. Thus complimentary reactions could be occurring on oppositesides of the fiber walls. Such reactions might be even further enhancedby the presence of catalytic materials on the internal wall surfaces orwhere the material comprising the fiber is itself catalytic to one ormore of the desired reactions.

The use of these hollow fibers in equilibrium reactions to shift theequilibrium in a desired direction also involves another form of gasdiffusion. In particular, it is effective for reactions which arelimited by equilibrium and have a small molecule reaction by-product,e.g., hydrogen. The equilibrium can be effectively shifted in thedirection of the product by the removal of this small molecule. Byemploying the fibers of the present invention, it is possible to operategas phase reactions at optimum pressures and still obtain a desirableconversion. Likewise it is possible to operate in temperature ranges ofless favorable equilibrium constants at which undesirable side reactionsmay be repressed or entirely eliminated. The processes contemplatedfurther permit the utilization of more economical operating conditions,including by way of illustration, adjustment of reactant concentrations,to obtain improved product yields and conversions as compared toconventional operations under comparable conditions in the absence ofgas diffusion. By reducing the small molecule, e.g., hydrogen,concentration in the gaseous mixture undergoing reaction, the overallequilibrium for the particular chemical reaction under considerationwill shift toward formation of additional reaction products (includinghydrogen); as a result, a more complete conversion of initial reactantsto products is obtained than could be realized in the absence of the gasdiffusion under similar reaction conditions.

In order to assist in more fully understanding the shifting ofequilibrium reaction processes as improved by the fibers of thisinvention, attention is directed to British Pat. No. 1,039,381 which ishereby incorporated by reference. Such processes are widely practiced onan industrial scale. For example, large quantities of hydrogen areproduced by steam reforming of hydrocarbons or methanol, by thermaldecomposition of hydrocarbons, by partial oxidation processes employinghydrocarbon feeds, and by the reaction of CO with water (steam). Otherknown gas-phase reactions, in which hydrogen is one of the products, arepracticed commercially, not primarily as a method for the commercialproduction of hydrogen, but as a result of which product hydrogen (inwhich case possibly better referred to as "byproduct" hydrogen) isproduced. For example, mention can be made here of specificdehydrogenation reactions such as the conversion of cyclohexane tobenzene, or of isopentane to isoprene, wherein the desired product isthe hydrocarbon, and hydrogen is a by-product. Hydrogenation reactionsmay also be performed utilizing the fibers of this invention.

Another example of such an equilibrium reaction is the dehydrogenationof ethyl benzene to styrene. This reaction normally takes place at 600°C. with a conversion of about 50%. By removal of the by-product,hydrogen, through the use of hydrogen diffusion through, for instance, ahydrogen diffusible metal hollow fiber of this invention, the reactioncan be shifted to effect greater productivity. The hollow fiber, ofcourse, being constructed to withstand the high temperatures.

Still another example of an equilibrium reaction would be thedehydrogenation of propionitrile to acrylonitrile. Propionitrile is aby-product of acrylonitrile manufacture. Normal dehydrogenationtechniques at elevated temperatures simply decompose the propionitrileto unwanted products. An effective dehydrogenation, however, can becarried out by homogeneous catalysis using metal complexes at 175° C.Unfortunately, the dehydrogenation is limited to a 1% conversion. Byremoving hydrogen by hydrogen diffusion using the hollow fibers of thisinvention, the equilibrium could be shifted favorably to effectincreased conversion.

Gaseous phase reactions wherein hydrogen is a product of the reactionare often effected in the presence of homogeneous or heterogeneouscatalysts, and the particular reactor employed for the practice of thisinvention can be provided with catalyst materials. For instance, wherehydrogen diffusion is effected within the reactor itself with a solidcatalyst, the reaction chamber can be packed with solid catalyst, thehollow fibers being in intimate contact with the catalyst so thathydrogen diffuses therethrough as soon as it is formed. Furthermore, forsuch reactions it is possible that the inorganic material, i.e., metal,may itself function as a catalyst or catalyst support, which whencoupled with the large available surface area within the fiber wallstructure provides particularly improved equilibrium reaction processes.

Hydrogen resulting from diffusion as described above is extremely pure.This is particularly desirable.

"Fuel cell" as used herein is a name commonly applied to anelectrochemical cell capable of generating electrical energy throughelectrochemical combustion of a fuel gas with an oxygen-containing gas.These cells have been fully described in the literature. Their preciseconstruction and operation does not form a part of the instant inventionexcept in an incidental capacity. However, a brief description of thenature and construction of a simple fuel cell is believed helpful, ifnot essential, in understanding the function and importance of theimprovement provided by the present invention.

In general, the simplest fuel cell comprises a housing, two electrodesand an electrolyte which acts as an oxygen transferring medium. Anoxidizing gas such as air under super-atmospheric pressure is circulatedon one side of the oxidizing electrode and a fuel gas, such as hydrogen,under super-atmospheric pressure is circulated on one side of the otherelectrode. A three-phase interface exists at each electrode, i.e., gas,electrolyte, and solid where a process of adsorption and de-adsorptionoccurs generating an electrochemical force. When current is drained fromthe two electrodes there is a net flow of electrons from the fuel gaselectrode through an external electrical circuit to the oxidizing gaselectrode. Thus, according to the external electron flow convention, theoxidizing gas electrode is the positive electrode and the fuel gaselectrode is the negative electrode. Oxygen is consumed at the positiveelectrode surface and fuel gas is oxidized into products of combustionas electrical energy while the remainder is released as heat.

With reference to FIG. 9, illustrative of a fuel cell, within casing 15is an electrolyte material 16, say a mixture of alkali carbonates, inwhich is positioned a cylindrically oriented supporting structure 17which supports the fuel and oxygen electrode element illustrated in FIG.8.

FIG. 8 illustrates a fuel and oxygen electrode element containing a fuelelectrode 1 which can be a hydrogen diffusible hollow fiber of thisinvention having an essentially non-porous skin, say, nickel-iron alloy,an oxygen electrode 2 which can be, for instance, silver tubing havingan outer diameter of about 508 microns and a wall thickness of about 102microns, maintained in relative arrangement by small ceramic sleeves 3.

Returning to FIG. 9, the fuel and oxygen electrode element is wound inhelical fashion around and supported by supporting structure 17 withfuel electrode inlet end 8 and oxygen electrode inlet end 9 displacedfrom the electrolyte material and provided with a source of hydrogen,e.g., hydrocarbon-water and methanol-water, and a source of oxygen,e.g., air. Exit ends 10 and 11 are also displaced from the electrolytematerial and are provided with means for undiffused gases to exit thecell. In operation at elevated temperatures, the fuel gas is fed intoinlet end 8 of the hollow fiber. Hydrogen diffuses through the fiberwall and undergoes the anode reaction on the surface side exposed to theelectrolyte material 16. With hydrogen forming fuel gases, because ofthe depletion of hydrogen, more hydrogen will be produced from the fuelgas in the bore of the fiber as it passes through the helical coil. Thehollow fiber acts as the anode which can be electrically connected to anegative lead 5. It should be noted that the hydrogen dissociates as itdiffuses through the fiber wall. Oxygen bearing gas is fed into inletend 9 and undergoes reaction at the electrolyte material surface, thetubing acting as the cathode which can be electrically connected topositive lead 6. In the electrolyte material the proton migrates to jointhe hydroxyl ion to form water which, due to the elevated operatingtemperature, i.e., 600° C., readily leaves the reaction zone. Currentflows through leads 5 and 6 when the cell is operated and the leads areconnected through load 7. Some advantages of such a cell are high powerand energy density and hydrogen available in an activated form. Thehollow fiber acts as its own current collector as does the silvertubing. There is no porosity problem nor is there an undue concentrationreduction of voltage. The fiber of, e.g., nickel, is resistant to moltenelectrolytes. There is no Carnot limitation of energy conversion. Theprincipal operating disadvantage is a requirement to operate at elevatedtemperatures.

The hollow fibers of the present invention comprising hydrogendiffusible metals and having an essentially non-porous compact layer areparticularly useful in such fuel cells.

Workers in the fuel cell field have used both porous and non-poroushydrogen diffusion membranes as the hydrogen, or fuel, electrode. Forinstance, U.S. Pat. No. 3,092,517 discloses the use of a thin non-porouspalladium-silver alloy membrane as the hydrogen diffusion electrode.Likewise, U.S. Pat. No. 3,332,806 discloses the use of thinpalladium-silver alloy foils supported by gold-nickel grid supports.U.S. Pat. Nos. 3,266,263 and 3,303,055 disclose porous fuel cellelectrodes that have varying porosity through the electrode. Theselatter electrodes are planar in construction. More recent U.S. Pat. No.3,981,749 discloses a planar gas diffusion electrode which has varyingporosity throughout its structure which is formed of a binding agent anda substance such as graphite, nickel oxide, aluminum oxide, or the likeis provided on the electrolyte side of the high porosity electrode. Thehollow fibers described herein are substantial improvements over theseefforts.

The ready application of specific catalyst material, if needed, to theinternal surface of the wall of the hollow fiber may allow smalleramounts of precious metal catalysts to be used. Further, the metalhollow fiber could be made of nickel or cobalt where the surfaces couldreadily be modified chemically for catalyst activity.

The use of air to provide oxygen for the fuel cell is anotherapplication of the hollow fibers of this invention. For such oxygenelectrodes it is desirable to have a large diffusion surface area; toseparate the oxygen from the nitrogen and carbon dioxide (to prevent theprecipitation of electrolyte carbonates); to have a catalytic surfacefor the oxidation or reduction of the oxygen; to extend the capabilityof the separation system to the temperature region useful for the oxygenelectrode; and to provide a current collecting surface for theelectrode.

All these objectives can be achieved with the hollow fibers of thisinvention. The catalytic element could be furnished on the surface ofthe fiber or, if economy permits, the catalytic element could be usedthroughout the fiber. In addition, a surface is provided to enhance theoxygen separation process. This could be the same metal as the catalystin the compact layer or it could be a separate metal or a suitablepolymer material placed on the fiber after formation.

One mode of operation would circulate air into the bore of the fiber.Some oxygen will diffuse through the essentially non-porous compactlayer to the outside of the fiber and the remaining nitrogen and carbondioxide could be discharged from the fiber bore. A slight depletion ofoxygen would occur in the air stream passing through the hollow fiberbore. The metal comprising the oxygen electrode hollow fiber, forinstance silver, would allow the electrode to operate at temperaturesbeyond the reach of polymeric hollow fibers. Silver and platinum can beused as catalysts in this electrode. The oxygen electrode describedwould also be useful in a fuel cell such as a methanol-oxygen cell.

A rather unique application for the essentially non-porous compact layerfibers of this invention is in the sodium-sulfur battery. In thisbattery, thin walled solid electrolytes, such as β-alumina, separate thesodium from the sulfur and have been found to be technically feasible.β-alumina as the sinterable inorganic material of the fiber of thisinvention provides excellent solid electrolytes for such batteries.

The invention is further illustrated by, but not limited to, thefollowing examples.

EXAMPLE 1

599 Grams of hematite (Fe₂ O₃), 500 grams of magnitite (Fe₃ O₄), and212.1 grams of an acrylonitrile copolymer (about 93% acrylonitrile andabout 7% vinyl acetate) were intimately mixed in a rod mill for 10hours. The hematite and the magnitite had an average particle size ofabout 1 micron and 0.7 micron, respectively. 850 cc of dimethylacetamideand 0.5 cc of a wetting agent (Tween 40) were mixed and chilled to +10°C. and placed into a large Waring Blender (Model No. 1112). The mixtureof oxide and polymer were transferred to the blender and stirred in byhand to give a reasonably uniform mixture. The mixture was chilled to+10° C. to reduce the solvency of the solvent and allow the polymer tobe dispersed mechanically with little going into solution. The blenderwas turned on high speed causing further blending of the oxide andcomplete dissolution of the polymer. The blender was turned off when atemperature of about 42.5° C. was obtained as determined by athermocouple in the mixture. The heat for the temperature rise beingsupplied by the degradation of mechanical energy. During the mixingperiod, a vacuum of about 56 cm of mercury was maintained over thecontents of the blender to reduce the amount of air entrapment in themixture. The resulting mixture was a solution of the acrylonitrilecopolymer containing a uniform dispersion of hematite and magnititeparticles.

This mixture was transferred to the dope pot of a spinning line having aspinneret immersed in a coagulating bath. Here the mixture was subjectedto a vacuum of about 56 cm of mercury for 0.5 hour. It was thenpressurized to 2.4 kg/cm² for 0.25 hour. A gear pump (Zenith pump, sizenumber one, hereafter a one capacity pump) rotating at 8.0 rpm delivered4.6 cubic centimeters of the mixture per minute. The mixture wasfiltered through a filter stack having a final stainless steel screen of120 mesh. The filtered mixture entered a hollow fiber spinneret havingan outer diameter of about 559 microns. The center pin of 240 micronsO.D. and 152 microns I.D. delivered water for inner coagulation at therate of 2 cc per minute through the center pin capillary. The extrudedfiber was externally coagulated in a coagulating bath held at 30° C. Thetemperature of the mixture in the dope pot was higher than thetemperature in the coagulating bath. The coagulating bath contained 50%,by volume, of dimethylacetamide and water.

The fiber was taken up at a first godet at 15 meters per minute andproceeded through the process at substantially this same rate. Leavingthe process, the precursor fiber was taken up on a bobbin using aLeesona winder. The bobbin from the spinning line was placed at theinput side of a furnace conversion system. A portion of the precursorfiber on this bobbin was fed into the furnace and converted at 1100° C.;reducing gases being fed into the exit end of the furnace at the rate of15 liters per minute. The reducing gas contained about 88.2% hydrogen,6.7% methane and 5.1% carbon monoxide.

The resultant iron fiber had a radially anisotropic internal void volumewall structure with an outer diameter of about 572 microns and an innerdiameter of about 173 microns.

At the peripheral internal zone the fiber wall structure is highlyfractured.

EXAMPLE 2

1000 Grams of black nickel (ic) oxide, a nickel oxide obtained fromFisher Scientific Co. as Fisher N-66, were mixed with 800 cc ofdimethylacetamide and 1.2 cc of Tween 40 (wetting agent). The mixturewas thoroughly mixed and agglomerates of the oxide were broken up in aWaring Blender for 0.5 hour. The contents of the blender were chilled to+10° C. 205 grams of an acrylonitrile copolymer (about 93% acrylonitrileand about 7% vinyl acetate) was added to the blender and premixed byhand to thoroughly wet the polymer and produce a reasonably uniformmixture. The blender was turned on to high speed causing further mixingof the oxide and complete dissolution of the polymer. The blender wasturned off when the temperature reached about 65° C. as determined by athermocouple in the mixture. The heat for the temperature rise beingsupplied by the degradation of mechanical energy. During the mixingperiod, a vacuum of about 56 cm of mercury was maintained over thecontents of the blender to reduce air entrapment in the mixture. Theresulting mixture was a solution of the acrylonitrile copolymercontaining a fine dispersion of nickel oxide particles.

This mixture was transferred to the dope pot of a spinning line having aspinneret immersed in a coagulating bath. The mixture was subjected to avacuum of about 56 cm of mercury for 0.5 hour. It was then pressurizedto 2.4 kg/cm² for 0.5 hour. A one capacity pump rotating at 12 rpmdelivered 7.0 cc per minute of the mixture. The mixture was filteredthrough a stack having a final stainless steel screen of 160 mesh. Thefiltered mixture entered a hollow fiber spinneret having an outerdiameter of about 1067 microns and an inner pin with an O.D. of about711 microns and an I.D. of about 406 microns. Water served as the innercoagulant and flowed at a rate of 0.62 cc per minute through the centerpin. The resulting extruded fiber was externally coagulated in a 45%dimethylacetamide, 55% water, by volume, coagulating bath held at 27° C.The temperature of the mixture in the dope pot was higher than thetemperature in the coagulating bath. The fiber was taken up at a firstgodet at 6 meters per minute and was washed with water at a second godetfollowed by stretching (2.5 fold) in boiling water between the secondand third godet. The fiber was relaxed at an 0.8 ratio between the thirdand fourth godet. Finally, the fiber was taken up at 12 meters perminute on a bobbin using a Leesona winder.

The precursor fiber, after drying on the bobbin, was placed at the inputside of a conversion furnace. A portion of the precursor fiber on thebobbin was fed into the furnace and converted at 1100° C.; reducinggases being fed into the exit end of the furnace at a rate of 14 litersper minute. The reducing gas consisted of 1.9% CO and the remainderhydrogen. Both the precursor fiber and the nickel fiber exhibited aradially anisotropic internal void volume wall structure having acompact layer at the fiber's external surface. The fiber has an outerdiameter of about 663 microns and an inner diameter of about 203microns.

EXAMPLE 3

A mixture of 500 grams of hematite (Fe₂ O₃), 500 grams of nickel (ic)oxide, and 250 grams of an acrylonitrile copolymer (about 93%acrylonitrile and about 7% vinyl acetate) were mixed in a rod millovernight. A mixture of 800 cc of dimethylacetamide and 1.2 cc of awetting agent (Tween 40) was chilled to +10° C. in a large WaringBlender. The mixture of oxides and polymer were transferred to theblender and stirred in by hand to give a reasonably uniform mixture. Theblender was turned on high speed causing further blending of the oxideand complete dissolution of the polymer. The blender was turned off whenthe temperature of the mixture reached about 42.5° C. During the mixingperiod, a vacuum of about 56 cm of mercury was maintained over thecontents of the blender to reduce air entrapment in the mixture. Theresulting mixture was a solution of the acrylonitrile copolymercontaining a fine dispersion of the nickel and iron oxide particles.

The mixture was transferred to the dope pot of a spinning line having aspinneret immersed in a coagulating bath. Here the mixture was subjectedto a vacuum of about 56 cm of mercury for 0.5 hour and then pressurizedto 2.4 kg/cm² for 0.5 hour. A one capacity pump rotating at 12.0 rpmdelivered 7.0 cc of the mixture per minute. The mixture was filteredthrough a filter stack having a final stainless steel screen of 120mesh. The filtered mixture was extruded as a hollow fiber through aspinneret having an outer diameter of about 635 microns with a centrallylocated hollow pin having 254 microns O.D. and 1.52 microns I.D. Waterserved as the inner coagulant and flowed at the rate of 5.0 cc perminute through the center pin. The fiber was coagulated in a 50%dimethylacetamide, 50% water, by volume, coagulating bath at 27° C. Thetemperature of the mixture in the dope pot was higher than thetemperature in the coagulating bath. The fiber was taken up at a firstgodet at 6 meters per minute and was washed with the coagulating bathmixture. The fiber was also washed with water on a second godet followedby stretching 2.5 fold, in boiling water, between the second and thirdgodet. The fiber was relaxed at an 0.8 ratio between the third andfourth godet and taken up on a bobbin using a Leesona winder at 12meters per minute. The precursor fiber, after drying on the bobbin, wasplaced at the input side of a conversion furnace. A portion of theprecursor fiber on the bobbin was fed into the furnace and converted to1100° C.; reducing gases being fed into the exit end of the furnace at arate of 14 liters per minute. The reducing gas consisted of 1.9% CO andthe remainder hydrogen. Both the precursor fiber and the nickel alloyfiber exhibited a radially anisotropic internal void volume wallstructure having a compact layer at the fiber's external surface. Thenickel alloy fiber has an outer diameter of about 559 microns and aninner diameter of about 173 microns.

EXAMPLE 4

128.8 Grams of sodium silicate (anhydrous), 28.8 grams of silicondioxide and 40.6 grams of calcium oxide were mixed into 600 cc ofdimethylacetamide.

The mixture was thoroughly mixed and agglomerates broken up in a WaringBlender for 0.5 hour. The contents of the blender were then chilled to+10° C. 135.9 grams of an acrylonitrile copolymer (about 93%acrylonitrile and about 7% vinyl acetate) were added to the blender andhand blended to give a reasonably uniform mixture.

The blender was turned on to high speed causing further mixing of theoxide and complete dissolution of the polymer. The blender was turnedoff when the temperature of the mixture reached 100° C. During mixing avacuum of about 56 cm of mercury was maintained over the contents of theblender to reduce air entrapment in the mixture.

The resulting mixture was a solution of the acrylonitrile copolymercontaining a fine dispersion of the sodium silicate, silicon dioxide andcalcium oxide particles.

This mixture was transferred to the dope pot of a spinning line. Herethe mixture was subjected to a vacuum of about 56 cm of mercury for 0.5hour. It was then pressurized to 2.4 kg/cm² for spinning having aspinneret immersed in a coagulating bath. A one capacity pump rotatingat 25 rpm delivered 14.6 cc per minute of the mixture. The mixture wasfiltered through a coarse screen of 80 mesh. The filtered mixtureentered a hollow fiber spinneret having an outer diameter of about 1321microns and a hollow center pin with an O.D. of 889 microns and an I.D.of 584 microns. Water served as the inner coagulant and flowed at 3.1 ccper minute through the center pin. The fiber was coagulated in a 45%dimethylacetamide, 55% water, by volume, coagulating bath at 27° C. Thetemperature of the mixture in the dope pot was higher than thetemperature in the coagulating bath. The spinning was intermittent butsamples of precursor fiber were obtained that had a radially anisotropicinternal void volume wall structure. A section of the precursor fiberwas passed through a furnace at 1100° C. for eight minutes in a nitrogenatmosphere. The section of precursor fiber turned black, probably due tothe presence of particles of carbon. This fiber sample was then heatedin the presence of air at 1000° C. for one hour. The resulting sectionof glass hollow fiber was hard, continuous having a radially anisotropicinternal void volume wall structure with a compact layer at the fiber'sexternal surface. The fiber has an outer diameter of about 1311 micronsand an inner diameter of about 1048 microns.

EXAMPLE 5

A mixture of 920 grams of nickel (ic) oxide (Fisher N-66), 80 grams ofmagnetite (Fe₃ O₄) (Fisher I-119) and 800 cc of dimethylacetamide wasplaced in a ball-mill containing steel balls. The ball mill was rununtil these materials were thoroughly mixed and the agglomerates andother large particles were essentially broken up. This mixture waschilled to approximately +10° C. and filtered through a Buckner funnel,which used a fine filter medium of 100% nylon filter fabric, Style No.W.N.H. -Y 7MO-PD8 (Feon). The steel balls were separated through a largescreen located above the Buckner funnel. The filter fabric removed anylarge particles or agglomerates which were not broken up during theball-milling.

The effluent from the filter flowed directly from the funnel into alarge Waring Blender. 204.8 grams of a copolymer of acrylonitrile (about93% acrylonitrile and about 7% vinyl acetate) was added to the blenderand premixed to produce a reasonably uniform mixture. (The solvent hadbeen chilled to allow premixing of the polymer without dissolution.) Theblender caused further mixing of the oxides and dissolution of thepolymer. The blending was completed when the temperature reached 75° C.as sensed by a thermocouple immersed in the mixture. The heat for thetemperature rise was supplied by the degradation of mechanical energyduring mixing. In this mixing period, a vacuum of about 56 cm of mercurywas maintained over the contents of the blender to reduce gas entrapmentin the mixture. The resulting mixture was a solution of theacrylonitrile copolymer containing a uniform dispersion of the oxideparticles.

This mixture was immediately transferred to the dope pot of a spinningline having a spinneret immersed in a coagulating bath. The mixture wassubjected to a pressure of 4.2 kg/cm², and pumping commenced. A onecapacity pump rotated at 6.0 rpm to deliver 3.5 cc of the mixture perminute. The mixture was filtered in line through a stack having a finalsteel screen of 400 mesh. The filtered mixture entered a spinneret forthe formation of the hollow fiber. This spinneret had an outer diameterof about 711 microns and a center pin of about 457 microns O.D. and 254microns I.D. The inner coagulant, which was water at 22° C., flowed at arate of 0.76 cc per minute through the center pin. The coagulating bath,at 18° C., contained a 65% dimethylacetamide and 35% water. Thetemperature of the mixture in the dope pot was higher than thetemperature of the coagulating bath. The fiber, after passing throughthe coagulating bath, was taken up at a first godet at a rate of 6meters per minute. It was washed on this godet with solution from thecoagulating bath (further aiding the coagulation process). The fiber waswashed on a second godet with deionized water. The fiber was thenstretched (2.5 fold) between the second and third godet in a stretchbath containing boiling water. After stretching, the fiber was relaxed(annealed) at a 0.8 ratio between the third and four godets. Finally,the fiber was taken up at 12 meters per minute on a bobbin using aLeesona winder. The polymeric precursor hollow fiber had an outerdiameter of about 643 microns, and an inner diameter of roughly 0.5 thisvalue.

The bobbin, containing the precursor fiber, was soaked for approximately18 hours in a container with a constant flow of fresh deionized water.After drying in air at room temperature and humidity for 24 hours(approximately 25° C. and 50% R.H.), the bobbin was placed at the inputside of a conversion furnace. The fiber was unwound from the bobbin in awater container prior to metering and steaming. The fiber then enteredthe furnace by means of a conveyor belt through a small gate opening.The furnace temperature was 1080° C. and was fed at a rate of 7.6 litersper minute with a gas whose composition contained about 34.4% hydrogen,0.9% carbon monoxide, and 64.7% nitrogen. The conversion time for theoperation was 8 minutes at the operating temperature.

The resulting nickel-iron alloy fiber, as did the precursor fiber, had awall structure having a radially anisotropic internal void volume and askin on the external surface. The fiber was tough and ductile. The fiberhad an outer diameter of about 381 microns and an inner diameter ofabout 203 microns.

During a test, with a reformer gas containing about 37% hydrogen and 51%water vapor with the remaining portions consisting of small amounts ofcarbon monoxide, carbon dioxide, and methane, the permeation rate foundfor hydrogen at various temperatures is shown in the following Table.

                  TABLE                                                           ______________________________________                                        Temperature Permeation Rate                                                   (°C.)                                                                              [cm.sup.3 (STP)/cm.sup.2 -sec-(cmHg).sup.0.5 ]                    ______________________________________                                        700         1.2 × 10.sup.-3                                             750         1.7 × 10.sup.-3                                             800         2.2 × 10.sup.-3                                             855         3.0 × 10.sup.-3                                             ______________________________________                                    

EXAMPLE 6

A mixture of 264 grams of β-alumina (calcined XB-2, Superground fromAlcoa Chemicals Company) and 600 cc of dimethylacetamide was placed in aball-mill containing ceramic balls. The mixture was milled forapproximately 100 hours to thoroughly mix the ingredients and break upagglomerates. The contents of the ball-mill was then transferred to alarge Waring Blender after separation of the ceramic balls. The contentsof the blender was chilled to -10° C. and an acrylonitrile copolymer(about 87% acrylonitrile, about 7% vinyl acetate and about 6% vinylbromide) was added together with 0.6 cc of a wetting agent (Tween 40).The resulting slurry was chilled to allow premixing of the copolymerwithout dissolution. The blender caused further mixing of the β-aluminaand dissolution of the polymer. Mixing was completed when a temperatureof 65° C. was reached. The heat for the temperature rise was supplied bythe degradation of mechanical energy during mixing. While mixing, avacuum was maintained over the contents of the blender to reduce gasentrapment in the mixture. The resulting mixture was a solution of theacrylonitrile copolymer containing a uniform dispersion of the β-aluminaparticles.

This mixture was transferred to the dope pot of a spinning line having aspinneret immersed in a coagulating bath. The mixture was subjected to apressure of 4.5 kg/cm² and pumping commenced. A one capacity pumpdelivered the mixture at a rate of 7.0 cc per minute. The mixture wasfiltered through a 60 mesh in-line filter. The filtered mixture entereda spinneret for the formation of a hollow fiber. This spinneret had anouter diameter of about 1067 microns with a center pin of about 711microns O.D. and about 406 microns I.D. The inner coagulating fluid,water at 22° C., flowed at a rate of 3.0 cc per minute through thecenter pin. The coagulating bath was a 50%, by volume, mixture ofdimethylacetamide and water at 21° C. The temperature of the mixture inthe dope pot was higher than the temperature in the coagulating bath.The coagulated fiber was taken up at a first godet at the rate of 6meters per minute and washed with the coagulating bath solution whichfurther aided coagulation. The fiber was washed with deionized water onthe second godet. The fiber was stretched (2.5 fold) between the secondand third godet in boiling water. To increase toughness, the fiber wasrelaxed (0.8 ratio) between the third and fourth godet in boiling water.Finally the fiber was taken up at 12 meters per minute on a bobbin usinga Leesona winder. A portion of the resulting fiber was soaked overnightin a 10% sodium carbonate solution and dried in a drying oven atapproximately 65° C. under about 56 cm of mercury vacuum for about 2hours. A section of this dried precursor fiber was covered with aluminapowder and heated under nitrogen to 1750° C. and held at thattemperature for 1 hour. The resulting hollow fiber, comprisingβ-alumina, and the precursor fiber exhibited a radially anisotropicinternal void volume wall structure having a compact layer at thefiber's external surface. The fiber has an outer diameter of about 599microns and an inner diameter of about 318 microns.

EXAMPLE 7

292 Grams of aluminum atomized powder (Reynolds Metals Co., grade 1-131)and 204.8 grams of a copolymer of acrylonitrile (about 93% acrylonitrileand about 7% vinyl acetate) were hand dispersed into 800 cc ofdimethylacetamide solvent previously chilled to +10° C. Thorough mixingof the aluminum powder and dissolution of the copolymer was carried outin a Waring Blender until a final temperature of 70° C. was reached. Theheat for the temperature rise was obtained from the degradation ofmechanical energy during mixing. The resulting mixture was transferredfrom the blender to the dope pot of a spinning line having a hollowfiber spinneret immersed in a coagulating bath. A one capacity pumpdelivered 7.0 cc of the mixture per minute to a spinneret. The spinnerethad an outer diameter of about 1829 microns with a center pin of 1245microns O.D. and 838 microns I.D. The inner coagulating fluid, suppliedto the center pin, was water at approximately 25° C. The coagulatingbath was a 65% dimethylacetamide, by volume, mixture with water at 18°C. The temperature of the mixture in the dope pot was higher than thetemperature in the coagulating bath. The coagulated fiber was taken upfrom the coagulating bath on a first godet at 6 meters per minute andwashed with the coagulating bath solution to further aid coagulation.The fiber was washed with deionized water on the second godet. The fiberwas then stretched (2.5 fold) between the second and third godet inboiling water. The fiber was then annealed (0.8 ratio) between the thirdand fourth godet in boiling water. Samples of the resulting precursorhollow fiber were taken from the fourth godet. These were examinedmicroscopically and found to have a radially anisotropic internal voidvolume wall structure.

A sample of the precursor fiber was placed in a tube furnace and allowedto heat up to 1000° C. in the presence of air. The sample was then heldat this temperature for two hours. After allowing the fiber to cool, theresulting aluminum oxide hollow fiber was examined and found to alsohave a radially anisotropic internal void volume wall structure. Thefiber has an outer diameter of about 823 microns and an inner diameterof about 404 microns.

While the invention has been described herein with regard to certainspecific embodiments, it is not so limited. It is to be understood thatvariations and modifications thereof may be made by those skilled in theart without departing from the spirit and scope of the invention.

What is claimed is:
 1. In a process for diffusing hydrogen through ahydrogen diffusible non-porous metal barrier, the improvement wherein ahydrogen diffusible metal, monolithic hollow fiber having a radiallyanisotropic internal void volume wall structure and an essentiallynon-porous compact layer comprises the non-porous hydrogen diffusiblemetal barrier.
 2. The process of claim 1 wherein the metal of the hollowfiber comprises nickel or nickel alloy.
 3. The process of claim 2wherein the metal is a nickel alloy comprising nickel and iron.
 4. Theprocess according to claim 1, 2 or 3 wherein the hydrogen diffuses froma gas mixture obtained from an equilibrium reaction.
 5. The process ofclaim 4 wherein the hydrogen diffuses through the hollow fiber from agas mixture obtained from a methanol-water reforming reaction whichproduces hydrogen.
 6. The process of claim 4 wherein the hydrogendiffuses through the hollow fiber from a gas mixture obtained from ahydrocarbon-water reforming reaction which produces hydrogen.
 7. Theprocess according to claim 1, 2 or 3 wherein the hydrogen diffusion istaking place in a fuel cell.
 8. In a hydrogen diffusion cell having ahydrogen diffusible non-porous barrier, the improvement wherein thehydrogen diffusible non-porous barrier comprises hydrogen diffusiblemetal, monolithic hollow fibers having a radially anisotropic internalvoid volume wall structure and an essentially non-porous compact layer.9. The hydrogen diffusion cell according to claim 8 wherein the hydrogendiffusible non-porous barrier comprises a bundle of metal fiberscomprising a multiplicity of cords having a plurality of such hollowfibers twisted together.
 10. In a fuel cell having a fuel electrode, theimprovement wherein the fuel electrode comprises a hydrogen diffusiblenon-porous barrier according to claim 8.